morphological unit of the plant, but represent results of its metamorphosis, so the cell-wall is equally unessential; and either the term "cell" must acquire a merely technical significance as the equivalent of morphological unit, or some new term must be invented to describe the latter. On the whole, it is probably least inconvenient to modify
the sense of the word " cell."The liistological analysis of animal tissues has led to results and to difficulties of terminology of precisely the same character. In the higher animals, however, the modifications which the cells undergo are so extensive, that the fact that the tissues are, as in plants, resolvable into an aggregation of morphological units, could never have been established without the aid of the study of development, which proves that the animal, no less than the plant, com mences its existence as a simple cell, fundamentally iden tical with the less modified cells which are found in the tissues of the adult.
Though the nucleus is very constant among animal cells, it is not universally present ; and among the lowest forms of animal life, the protoplasmic mass which represents the morphological unit may b3, as in the lowest plants, devoid of a nucleus. In the animal, the cell-wall never has the character of a shut sac containing cellulose ; and it is not a little difficult, in many cases, to say how much of the so- called " cell-wall" of the animal cell answers to the " pri mordial utricle " and how much to the proper " cellulose cell-wall " of the vegetable cell. But it is certain that in the animal, as in the plant, neither cell-wall nor nucleus are essential constituents of the cell, inasmuch as bodies which are unquestionably the equivalents of cells true morphological units are mere masses of protoplasm, de void alike of cell-wall and nucleus.
For the whole living world, then, it results : that the morphological unit the primary and fundamental form of life is merely an individual mass of protoplasm, in which no further structure is discernible ; that independent living forms may present but little advance on this structure ; and that all the higher forms of life are aggregates of such morphological units or cells, variously modified.
Moreover, all that is at present known tends to the con clusion, that, in the complex aggregates of such units of which all the higher animals and plants consist, no cell has arisen otherwise than by becoming separated from the pro toplasm of a pre-existing cell; whence the aphorism "Omnis cellula e cellula."
It may further be added, as a general truth applicable to nucleated cells, that the nucleus rarely undergoes any considerable modification, the structures characteristic of the tissues being formed at the expense of the more super ficial protoplasm of the cells ; and that, when nucleated cells divide, the division of the nucleus, as a rule, precedes that of the whole cell.
2. In the course of its development every cell proceeds f rom a condition in which it closely resembles every other cell, through a series of stages of gradually increasing divergence, until it reaches that condition in which it presents the characteristic features of the elements of a special tissue. The development of the cell is therefore a gradual progress from the general to the special state.
The like holds good of the development of the body as a whole. However complicated one of the higher animals or plants may be, it begins its separate existence under the form of a nucleated cell. This, by division, becomes con verted into an aggregate of nucleated cells : the parts of this aggregate, following different laws of growth and multiplication, give rise to the rudiments of the organs ; and the parts of these rudiments again take on those modes of growth and multiplication and metamorphosis which are needful to convert the rudiment into the perfect structure.
The development of the organism as a whole, therefore, repeats in principle the development of the cell. It is a progress from a general to a special form, resulting from the gradual differentiation of the primitively similar mor phological units of which the body is composed.
Moreover, when the stages of development of two animals are compared, the number of these stages which are similar to one another is, as a general rule, proportional to the closeness of the resemblance of the adult forms ; whence it follows that the more closely any two animals are allied in adult structure, the later are their embryonic conditions distinguishable. And this general rule holds for plants no less than for animals.
The broad principle, that the form in which the more complex living things commence their development is always the same, was first expressed by Harvey in his famous aphorism, "Omne vivum exovo," which was intended simply as a morphological generalization, and in no wise implied the rejection of spontaneous generation, as it is commonly supposed to do. Moreover, Harvey s study of the development of the chick led him to promulgate that theory of " epigenesis," in which the doctrine that develop ment is a progress from the general to the special is im plicitly contained.
Caspar F. Wolff furnished further, and indeed conclu sive, proof of the truth of the theory of epigenesis ; but, unfortunately, the authority of Haller and the speculations of Bonnet led science astray, and it was reserved for Von Baer to put the nature of the process of development in its true light, and to formulate it in his famous law.
3. Development, then, is a process of differentiation by which the primitively similar parts of the living body become more and more unlike one another.
This process of differentiation may be effected in several ways.
(1.) The protoplasm of the germ may not undergo divi- tlon sion and conversion into a cell aggregate ; but various parts of its outer and inner substance may be metamorphosed directly into those physically and chemically different materials which constitute the body of the adult. This occurs in such animals as the Infusoria, and in such plants as the unicellular Alyce.
(2.) The germ may undergo division, and be converted into an aggregate of cells, which cells give rise to the tissues by undergoing a metamorphosis of the same kind as that to which the whole body is subjected in the preceding case.
The body, formed in either of these ways, may, as a whole, undergo metamorphosis by differentiation of its parts, and the differentiation may take place without reference to any axis of symmetry, or it may have reference to such an axis. In the latter case, the parts of the body which become dis tinguishable may correspond on the two sides of the axis (bilateral symmetry), or may correspond along several lines parallel with the axis (radial symmetry).
The bilateral or radial symmetry of the body may be further complicated by its segmentation, or separation by divisions transverse to the axis, into parts, each of which corresponds with its predecessor or successor in the series.
In the segmented body, the segments may or may not give rise to symmetrically or asymmetrically disposed pro cesses, which are appendages, using that word in its most general sense.
both animals and plants, is attained by the body when it becomes divided into segments provided with appendages; when the segments not only become very different from one another, but some coalesce and lose their primitive distinct
ness ; and when the appendages and the segments into