Evolution of Life/Embryology
EMBRYOLOGY.
The study of the transitional stages through which plants and animals pass from the early to the mature condition is not only of immense importance to the Physiologist, but equally so to the Zoologist, Botanist, and Geologist. Notwithstanding that some knowledge, at least, of Embryology is demanded in the study of Biology, the subject is comparatively little cultivated, owing, probably, to the limited means of obtaining material, and the difficult manipulation required in this kind of work. Nevertheless, since 1759, the year in which Wolff published his "Theoria Generationis" there have appeared from time to time Embryologists like Von Baer, Schleiden, Schwann, Coste, Remak, Rathke, etc., who, after overcoming the difficulties inherent to the nature of their studies, left treatises which will always be models of scientific work and philosophic thought. Our prescribed limits only permit of briefly calling attention to some of the conclusions of Embryology, pointing out the manner in which they confirm the theory of the evolution of life as deduced from the structure and petrified remains of the vegetal and animal kingdoms. Those who are ignorant of the early stages of plants and animals will hardly believe that beings so different as sea-weed, oaks, star-fishes, mollusca, guinea-pigs, rabbits, dogs, and men begin their life in the same way; yet Fig. 160 represents equally well the cell, or primitive stage, of any of the plants or animals just mentioned. Confining ourselves for the present to the animal kingdom, let us examine the cell, or the egg, of a mammal,—that of a rabbit (Fig. 160), guinea-pig, or man, for example. The egg of a mammal, about the of an inch in diameter, when magnified, is seen to consist of a cell-wall or Vitelline membrane (Fig. 160), inclosing cell-contents, or the Vitellus, in which is found the nucleus (Fig. 160, n), or Germinal vesicle, with its nucleolus, or Germinal spot. Let us observe what takes place, supposing the conditions to be favorable to the development of the egg. According to some observers, the Germinal vesicle and spot disappear; equally good observers, however, state that the Germinal vesicle and spot divide into two. While there is some doubt as to the disappearance of the Germinal vesicle and spot, all observers agree that the Vitellus, or cell-contents, divide into two segments (Fig. 161), and that each segment has its nucleus and nucleolus. As the segments are the halves of the Vitellus, probably the nuclei and nucleoli are formed through the division of the Germinal vesicle and spot. However this may be, the Vitellus divides into two segments, each segment having a nucleus with its nucleolus. These two segments subdivide into four balls (Fig. 162), the four into eight (Fig. 163), the eight into sixteen (Fig. 164), and so on. Through this process of cell-division, or segmentation, as it is called, the Vitellus is divided into a number of little balls, and assumes the shape of a mulberry. Finally, the superficial balls of the mulberry are transformed into cells, and so arrange themselves as to present the appearance of a mosaic pavement (Fig. 165); as the deeper balls become cells, they pass to the surface and increase the thickness of this mosaic-like membrane. In this manner the Vitellus is converted into a vesicle; within this vesicle there shortly appears a second vesicle; these two vesicles are usually called the Germinal layers, or the
External and Internal blastodermic membranes. If the 
egg in a slightly more advanced stage be now examined from a horizontal point of view, there will be seen a light oval space, the area pellucida, surrounded by a dark space, the area opaca (Fig. 166); within the area pellucida will be noticed an oval body, the Primitive trace, so called from indicating the position of the embryo, the furrow in the Primitive trace being known as the Primitive groove. A little later the Primitive trace and area pellucida become guitar-shaped (Fig. 167), and if a longitudinal section of the egg be examined (Fig. 168) it will be seen to consist of the External and Internal blastodermic membranes, and a third membrane lying between these two. The partial fusion of these membranes makes the Primitive trace, While these three membranes are consolidating into the Primitive trace, the Middle membrane splits into two layers: the Upper terminates in the External blastodermic membrane, the Lower grows gradually around the Internal blastodermic membrane, finally inclosing it. The embryo at this period is a guitar-shaped body (Fig. 177), consisting simply of three membranes lying over one another, narrowly bound together. The question may be asked by some of our readers. What relation does so minute a structure as the egg of a mammal bear to that of a bird? Does the development of a rabbit resemble that of the chick? The egg of a chicken (Fig. 174), as all the world knows, is composed of a shell inclosing a semi-liquid substance, in which is suspended a yolk. If a freshly-laid egg be carefully examined, however, supposing the conditions to have been favorable to development, there will be found lying on the top of the yolk a delicate sheath (Fig. 174, b), which is composed of two membranes; while the yolk itself, if laid open, exhibits in its interior a whitish body (Fig. 174, a), which, narrowing into a thread, runs upwards towards the membrane composing the sheath. This whitish substance is called the white yolk, as distinguished from the yellow yolk surrounding it. We have tried to explain how, by a continued process of cell-division, the contents of the egg of a mammal assume a mulberry-shaped form, and the gradual conversion of this mulberry into the External and Internal blastodermic membranes. If the yolk of the chicken be examined before it is surrounded by the semi-fluid substance (Fig. 173) and shell, there will be found lying on the top of the yellow yolk a membrane (Fig. 173, b) in which may be seen the Germinal vesicle and Germinal spot; by a process of cell-division, known as partial segmentation, this membrane is transformed into a heap of balls which gradually assume the form of the two membranes which, we have stated, are found lying upon the yellow yolk of the freshly-laid egg. In the course of development a third membrane appears between these two. The partial fusion of these membranes makes the Primitive trace, which passes from the oval to the guitar-shaped form (Fig. 177). The Middle membrane splits into two layers (Fig. 175), the Upper uniting with the External blastodermic membrane, the Lower bending down on the Internal blastodermic membrane (Fig. 176, c). The Middle and Internal blastodermic membranes now grow gradually downward around the yellow yolk, and finally inclose it. At this stage the embryo chick corresponds to Fig. 177, representing the embryo of a mammal. We see, therefore, that the development of the chick and the mammal is the same, while the difference between their eggs is not an essential one,—the nutriment for the mammalian egg being furnished from time to time, while that for the bird's egg is supplied at once in the form of yolk. A homely illustration of this difference is that of a man who receives his yearly food from day to day, and of one who receives his yearly food at once. The only part of the chicken's egg which corresponds to the mammal's egg is the membrane with its Germinal vesicle and Germinal spot, lying upon the yellow yolk of the unlaid egg.[1] Whatever view be taken of the relations of the eggs of the Vertebrata, the important point to be noticed is that the embryo of a fish, batrachian, reptile, bird, or mammal, including man at an early stage of life, is a guitar-shaped body (Figs. 177, 167), consisting of three membranes lying over one another, and narrowly bound together (Fig. 168); and if we were ignorant of the animal whose egg had been transformed into such a body, it would be very often impossible to say what would result from its development. These membranes are called blastodermic, or tissue germinating from the organs of the future animal growing in them.[2]
The skin and central nervous system are developed in the External, or upper membrane; the osseous, muscular, vascular, reproductive, and urinary systems, the walls of the alimentary canal, and its appendages, are produced in the Middle membrane; while the epithelium, which lines the alimentary canal and its appendages, the lungs, liver, etc., is derived from the Internal or lower membrane.
In speaking of the Primitive trace, at page 127, we called attention to the furrow known as the Primitive groove. As development proceeds, this furrow deepens, and if the embryo be viewed in transverse section (Fig. 175), this deepening is seen to be produced through the rising up of the External blastodermic membrane (Fig. 175, a) in two heaps, called Laminae Dorsales (Fig. 175, L), which, growing towards each other, finally coalesce, thus converting the Primitive groove into a tube (Fig. 176, K). This tube is the rudimentary central nervous system. Directly underneath this tube, in the Middle membrane, however, is seen a cylindrical rod of cells, the Chorda Dorsalis (Fig. 176, v), in which are developed the bodies of the vertebrae (segments of spine). By looking at Figs. 169 to 172 (Dog or Man), we see how, by a continually constructing process, the upper portion of the Internal blastodermic membrane (Fig. 169, I), with that part of the Middle membrane lying upon it, is gradually pinched off from the lower (Fig. 169, a), until, finally, only a narrow pedicle connects the two. The upper pinched-off portion is the primitive alimentary canal (Fig. 170, I), the lower the umbilical vesicle, or yolk-bag. The umbilical vesicle (Figs. 169 to 172, a), in the course of development, passes away, the time of its disappearance varying in different animals: thus, in the Trout it is retained till the sixtieth day. By referring to Figs. 169, 172, it will be seen that the alimentary canal and umbilical vesicle are composed of two layers. The inner layer, or the Internal blastodermic membrane, develops the epithelium of the mucous membrane; the outer layer, or the lower half of the Middle membrane, makes the wall of the alimentary canal. This is a very important distinction, since the lower lungs, etc., which first appear as buds sprouting from the alimentary canal, exhibit the same structure. In the Batrachia (Frog), and some Fishes, however, the whole of the Internal blastodermic membrane, with that part of the Middle membrane lying upon it, is used up in the formation of the alimentary canal, which is developed in a different manner from that of the dog or man; there is, therefore, no umbilical vesicle or yolk-bag. The gelatinous mass which surrounds the egg of the Frog furnishes the nutriment for the embryo. The development of the Reptile, Bird, and Mammal offers a striking contrast as compared with that of the Fish and Batrachian in the formation of the Amnion and Allantois. The External blastodermic membrane, at that point where the upper part of the Middle membrane unites with it, rises up into two folds (Fig. 169, d). These folds grow towards each other, arching over the embryo, and finally unite (Fig. 170). The inner fold then separates from the outer, and forms the Amnion (Fig. 171, d), while the outer fold recedes from the Amnion until it reaches the Vitelline membrane, with which it unites. These united membranes are known as the Chorion (Fig. 171, Ch). The Amnion becomes filled with the Amniotic fluid, in which the embryo is suspended. During the formation of the Amnion there buds out from the posterior portion of the embryo a sac (Figs. 169 to 172), which, in expanding, finally comes in contact with the Chorion, This sac is called the Allantois, and serves in Birds and Reptiles as a respiratory organ, the porosity of the egg-shell allowing the oxygen to pass in and the carbonic acid to pass out. In the Mammals, through the Allantois, the embryo is put in communication with the mother. We have now explained as briefly as possible the development of a vertebrate.
In the hatching process the Chorion, Allantois, and Amnion break, they being only temporary structures. It will be seen, therefore, that the animal is formed of but a portion of the three blastodermic membranes. Beginning alike in the form of a cell or egg, the Invertebrata and Vertebrata grow for some time in the same manner. As development advances, characteristic structures appear in the embryo, and the division, class, or order to which the future animal will belong becomes evident. Figs. 178 to 181, representing the embryo Turtle, Chicken, Dog, and Man, illustrate the resemblance of vertebrate animals at an early stage of their existence.' Not only, however, does man at such a period resemble a Turtle, and is undistinguishable from a Dog, but the transitory stages of his internal organization are also more or less represented as permanent structures in the lower animals. This generalization, which is one of the most important in Biology, may be expressed in the statement, that the structures which are transitory in the higher animals are retained permanently in the lower. Thus, for example, the spine of the higher animals is composed of a number of bony segments or vertebrae. These are represented in the embryo, however, by a cylindrical rod of cells, the Chorda Dorsalis, and by a few quadrate masses lying on each side of the central nervous system. The Chorda Dorsalis, which is only the rudimentary condition of the bodies of the vertebrae, is retained permanently, however, in the Amphioxus and Myxinoid fishes. The Chorda Dorsalis, until recently, was supposed to characterize the Vcrtebrata, and as it is a very important structure, its apparent absence in the Invertebrata (animals without a backbone) was often urged as an insuperable objection to the view of the higher forms of life having come from the lower. The free-swimming embryos of the Ascidian worms, however, according to Kowalebsky and others, exhibit, in their organization, a Chorda Dorsalis (Fig. 38 a, C) and a Central nervous system, which develop in the same manner as that observed in the Amphioxus, the simplest of fishes. The importance of this discovery cannot be exaggerated, as the embryo Ascidian furnishes the transition from the Invertebrata to the Vertebrata. We have seen that the Central nervous system is formed through the conversion of the Primitive groove into a tube. The tube is originally pointed at both ends, and this rudimentary condition is retained permanently in the spinal marrow of the Amphioxus (Fig. 40),—the fish without skull or brain. In all other Vertebrata, however, the anterior part of the spinal marrow, in the course of development, expands into a vesicle, which subdivides into three; the anterior of these three vesicles divides into two, and the posterior into two, the middle remains undivided; thus five vesicles (Fig. 177, 1, 2, 3, 4, 5) are formed out of the swelling of the anterior portion of the spinal marrow. These vesicles are called, translating their German names literally, the Fore brain, Between brain. Middle brain, Hind brain, and Hindmost brain, the different parts of the brain being developed from them. The brain of adult man, although highly complex in its organization, is nevertheless represented, at an early period of life, by five vesicles, being undistinguishable from those of an embryo dog, rabbit, bird, or fish. In fishes like the Myxine and Lamprey the brain remains in this undeveloped condition, thus exhibiting permanently the stages of the brain that are transitory in the higher animals. Every one knows that in breathing the air passes through the windpipe to the lungs, and that the food goes to the stomach through a separate and distinct tube. If, however, a Garpike be examined, its lung-like air-bladder is seen to communicate with the alimentary canal by a tube, the air-duct. This arrangement represents perfectly the rudimentary condition of the lungs in the human being, or in the embryo of the higher animal, as in these the lungs are developed as buds from the alimentary canal, the pedicle by which they are attached to it becoming later the windpipe, which corresponds to the air-duct of the Gar. The organs of Respiration naturally suggest those of Circulation. The successive stages through which the heart and blood-vessels of mammals pass in the course of development are more or less well represented by the vascular apparatus of the fish, batrachian, reptile, and bird. The termination of the Digestive, Reproductive, and Urinary apparatus in a Cloaca, exhibited in the embryo of man, is a permanent arrangement in the Sloth, Monotremata, Birds, and Reptiles. Finally, the development of the Skull and Extremities illustrates the same principle of the lower forms of life, representing the undeveloped stages of the higher. Has the Biologist any theory to offer as an explanation of these facts? One may reasonably ask, Why do the flipper of the seal, the foot of the turtle, the wing of the bird, the hoof of the horse, the claw of the lion, the hand of man, etc., develop from a bud? Why are these structures, used for such different purposes, constructed on essentially the same plan? Is there any explanation of the fact that man and the lower animals are undistinguishable in the early stages of their existence, and that the transitory phases through which man passes in the course of development are more or less permanently represented in the lower animals,—that is, that man is not absolutely at any time a Reptile or Dog, etc., but at a certain period exhibits an organization which is undistinguishable from that which later becomes a Turtle or Dog, etc.? It seems to us that the theory of the higher animals having descended from the lower explains perfectly all these facts. We will try to illustrate this view by noticing the effects supposed to be produced on the posterity of a family by their dispersion. After the lapse of ages, subjected to different conditions of soil, food, and climate, the races descending from this family would differ so greatly as regards their appearance, language, and customs that an Ethnologist might doubt if indeed they had come from one stock. If, however, he compared young individuals of these races, and found they resembled one another, and at an extremely early period of life were even undistinguishable, and, further, that sometimes individuals appeared that differed greatly from the race from which they descended, resembling rather a remote, often more barbarous, one; and, finally, that the individuals of a barbarous race, when subjected to more favorable conditions, in becoming more civilized, begin to resemble those more advanced,—considering these facts together, the view might be suggested to the Ethnologist that the different races had come from one stock. An important fact to be remembered in reference to the origin of races is that peculiarities which appear in the parent reappear in the offspring at the same age in which the parent was affected. Thus, the parent at a certain age develops a disease: his child grows up apparently healthy; suddenly the same disease appears in the child, and at the same age at which the parent was affected. Though the causes of peculiarities appearing in the parent, and the inheriting of them by the offspring, are still unknown, or very obscure, nevertheless we know it to be a fact that peculiarities—good or bad—affecting a parent may, and often do, reappear in the offspring in the manner just illustrated. Suppose, now, in a remote past, two animals, the descendants of the same parent, grew for some time alike, but that gradually they began to differ, acquiring certain peculiarities. These, if transmitted to posterity, would appear at the same age in which they were acquired by the parents. This hypothetical case illustrates what we suppose to have been the development of the Bird and Mammal from a common ancestor, the Reptile. This reptilian ancestor had two descendants; one acquired the peculiarities of the Bird, the other those of the Mammal: the Bird and Mammal of the present day ought, therefore, to develop in a reptilian manner until they attain the age at which their progenitors acquired the characteristic of the Bird and Mammal; from that time their development ought to be different. Our brief résumé of development shows that the facts perfectly confirm such a theory. By the same reasoning we conclude that the Reptiles and Batrachia have diverged from a form like that of the Lepidosiren, or Mud-fish, the Bony fish and Lepidosiren from the Ganoids, the Fish and Ascidians from some Sac-worm, the Echinodermata and Articulata from the Articulated Worms; finally, that the animal and vegetal kingdoms are the diverging stems of an intermediate kingdom, arising through spontaneous generation, or whose origin is unknown. This theory of the gradual descent of the higher animals from the lower explains perfectly why the phases exhibited in the development of man should be more or less permanently represented by lower animals, or, as John Hunter expressed it, "If we were to take a series of animals from the most imperfect to the perfect, we should probably find an imperfect animal corresponding with some stage of the most perfect." This view of nature throws light on the presence of rudimentary organs, such as the wings of birds and insects who never fly, the eyes of fish who, living in dark caves, never see, and the teeth of young birds and of certain whales who, when adult, do not have a tooth in their head. In the lung-breathing Vertebrata a right and left lung are usually present; the organization of the snakes and snake-like lizards exhibits the peculiarity of only one lung being developed, the other being rudimentary. Of the egg-sacs, or ovaries, of most birds, only the left is developed, the right being without function. Assuming the theory of the transmutation of species to be true, these rudimentary organs have a meaning, as indicating the ancestry of the animals exhibiting them. Important to the Evolutionist are, therefore, such structures as the plica semilunaris of the human eye, the representative of the third eyelid of lower animals, the external muscles of the human ear, the coccygeal bones composing the short tail of man, the vermiform appendix, etc. The monstrosities of the animal and vegetal kingdom are explainable from this point of view, the monstrosity usually consisting in the excessive development or deficiency of one or more organs, the abnormal in one animal being normal in another. Occasionally we find animals so badly organized as to make it incredible that they should have appeared on the earth as such originally. In speaking of the Sloth, Cuvier observes, "One finds them so little related to ordinary animals, the general laws of living organizations apply so little to them, the different parts of their body seem to be so much in contradiction with the rules of co-existence that we find established in the whole animal kingdom, that one could really believe that they are the remains of another order of things." Cuvier then continues by saying that in most forms the disadvantages are compensated by advantages, "but in the Sloth each singularity of organization seems to result only in feebleness and imperfection, and the inconveniences belonging to the animal are not compensated by any advantage."[3]
The only explanation, at present, of the existence of such a wretched animal as the Sloth is that it is the degenerated representative of some extinct animal who lived at the same time as the Megatherium, which it resembles in the form of its head. The limbs and backbone of the Megatherium are, however, represented by the Great Ant-eater. The Sloths and Great Ant-eater are confined to South America, and it is there that the Megatherium remains have been found in great abundance.
The development of the flower through the gradual metamorphosis of the leaf is a beautiful illustration of the evolution of different forms from a common type. In the words of Prof. Gray, "The leaves of the stem, the leaves or petals of the flower, and even the stamens and pistils, are all forms of a common type, only differing in their special development; and it may be added that, in an early stage of development, they all appear nearly alike. That which, under the ordinary laws of vegetation, would have developed as a leafy branch, here develops as a flower; its several organs appearing under forms some of them slightly and others extremely different in aspect and in office from the foliage. But they all have a common nature and a common origin, or, in other words, are homologous parts. When, therefore, the floral organs are called modified or metamorphosed leaves, it is not to be supposed that a petal has ever actually been a green leaf, and has subsequently assumed a more delicate texture and hue, or that stamens and pistils have previously existed in the state of foliage, but only that what is fundamentally one and the same organ develops, in the progressive evolution of the plant, under each or any of these various forms." The visceral arches of the Vertebrata are among the many illustrations of this idea offered by the animal kingdom. The visceral arches (Figs. 178 to 181, c) consist of thickenings or papillai situated behind the primitive eye, and below the primitive ear. They are present in the early stages of all Vertebrata, and are much modified in the course of development. The branchial arches supporting the gills in Fishes represent best their primitive condition, while in remaining Vertebrata they are used partly in the formation of the lower jaws, partly in the formation of the organs of hearing.
The subject of Embryology is as intimately related to Geology as to that of Anatomy, for the changes through which plants and animals pass in the course of development are essentially the same changes through which life in general has passed from its first appearance to the present time, for not only are the transitory stages of the higher animals permanently represented by the lower, but they are also permanently represented by the fossils. In other words, the development of the most complex plant, and of the most highly-organized animal, is an epitomized history of vegetal and animal life in general. Let us illustrate by a few examples. We stated in the chapter on Zoology that the Horse, Tapir, and Rhinoceros formed a natural group, they being connected through intermediate forms, the series beginning with the Paleotherium (Fig. 151). In the chapter on Geology we called attention to the fact that the Paleotherium appeared before the Horse, etc. The embryo Plorse, however, in his three toes and the structure of his teeth, represents the Paleotherium (Fig. 154), while the transitory stages through which the Horse passes from the Paleotheroid condition to its adult state are permanently retained in the Anchitherium and Hipparion. (Fig. 155). In Ruminating animals, like the Gazelle, Sheep, and Ox, the upper jaw is without incisor and canine teeth (Fig. 157); these exist, however, in a rudimentary condition in the embryos of these animals. The embryos also exhibit two distinct metacarpal and metatarsal (Fig. 158) bones, which, in the course of development, fuse into the so-called cannon-bone of the fore and hind leg (Fig. 159). Now, in the early part of the Tertiary period there lived animals like the Dichobune, Dichodon, and Anoplotherium (Fig. 152), whose adult organization represents very well the transitory stage of the hollow-horned Ruminants, the Anoplotherium having well-developed canine and incisor teeth, and retaining the condition of two distinct metacarpal and metatarsal bones (Fig. 158). The stages through which one of our hollow-horned Ruminants passes give thus a picture of the transitional stages through which the Ruminant order in general has passed. The molar teeth of these animals, as well as those of the Rhinoceros, Horse, etc., are very interesting from the Evolution point of view. The type of tooth characteristic of the Paleotherium runs, more or less modified, through the Rhinoceros, Tapir (Fig. 153), and Horse, while that of the Anoplotherium can be traced through the Hog, Hippopotamus, Sheep, Deer, etc.; but in still earlier forms, like Coryphodon, Pliolophus, and Lophiodon, we find teeth combining the characteristics of the Paleotherium and Anoplotherium. So that just as the Rhinoceros and Horse are specialized forms of the Paleotherium, the Pig and Sheep of the Anoplotherium, so the Paleotherium and Anoplotherium are specialized forms of the Coryphodon. Accepting the theory of the specialized higher forms of life having descended from a more general lower form, we have an explanation of the harmony offered by the anatomy, embryology, and petrified remains of these animals. But the theory of Evolution explains not only the most important facts in reference to this particular order of animals, but we hope to have shown that it is equally applicable to the whole vegetal and animal kingdom. The question now naturally arises, Are there any natural causes sufficient to effect the development of the animal and vegetal kingdoms out of a monad?
To that subject we now turn.
- ↑ We have called attention to the distinction of white and yellow yolk, as the white yolk, or part of it, is supposed by Peremesko to form the Middle layer of the chick; his view being that the balls of the white yolk, by an amoebiform movement, pass up and between the External and Internal blastodermic membranes, coalesce, and so form the Middle membrane.
- ↑ By many Physiologists the embryo is stated as consisting of two layers, the External and Internal germinal layers, or blastodermic membrane, from which the future animal is developed. The view of the embryo consisting of three germinal layers was distinctly enunciated by Remak as long ago as 1852, "Comptes Rendus," tome xxxv., and even earlier. Since that time Remak's views have been confirmed by Rathke, Kölliker, Strieker, Waldeyer, Klein, and others. We, therefore, give in the text what may be called the German theory of Embryology.
- ↑ Cuvier, "Ossemens fossiles."