is composed of a viscid semi-fluid material termed protoplasm, sur- rounded by an elastic membrane termed a cell wall. This wall is composed of cellulose in the case of the vegetable cell and may ac- quire considerable thickness and firmness; in the animal cell it is almost always very thin, and never consists of cellulose: but ap- pears to agree in composition with the protoplasm, of which it may be considered a condensation. There is reason to believe that in most if not all cases the cell wall is porous, and that adjacent cells are connected with one another by bridges of protoplasm.
The activities of living cells are manifested in four ways, viz. :
(1) By movement, which takes the form of alternate contraction and expansion.
(2) By secretion, i.e. by the production of definite substances which accumulate in the protoplasm, but which are finally extruded from the cell.
(3) By the transmission of stimuli, so that shocks or stimuli of various kinds applied to one cell result in movement or secretion in a different cell.
(4) By reproduction, which takes the form of the division of the cell into two daughter cells.
All these activities are dependent for their continuance on the consumption of a certain portion of the protoplasm ; and if life is to persist and if the cells are to increase in number this waste must be repaired. This is effected by the taking in and assimilation of food ; and this assimilation can only be carried out if there exists in the protoplasm a specially differentiated portion termed the "nucleus," which appears to be of rather different composition from the rest. For this reason it is customary to use the term "cytoplasm" for the extra-nuclear protoplasm.
Cells may be specialized for rapid movement or for the reception and transmission of stimuli or for secretion. Locomotor cells may be provided with freely projecting filaments, termed "cilia" or " flagella," which are capable of rapid vibration, and act the part of oars in propelling the animal, or their cytoplasm may contain threads termed " myonemes"or" muscular fibrils," which are capable of thickening and contracting; and these threads may be simple or cross-striped i.e. composed of alternate discs of singly and doubly refracting material; cells provided with these threads are termed " muscle cells."
Cells devoted to the reception and transmission of stimuli are known as " nerve cells " or " neurones." Those which are situated at the free surface of the animal, and which are provided with stiff processes of various kinds which are easily affected by stimuli, are known as " sense cells." Those which are more deeply situated have, in place of the stiff processes, one or more root-like branching pro- cesses known as "receptive dendrites " Stimuli when received are transmitted by outgrowths of the neurone known as "axons," and these axons, after preservation by fixing fluids, are seen to consist of a series of delicate fibres, the " neuro fibrillae." The axon ends in a brush of root-like processes called "terminal dendrites"; these den- drites are in contact either with the receptive dendrites of another neurone to which they hand on the stimulus, or with a muscle cell in which they cause contraction, or with a secretory cell in which they initiate secretion.
In the case of secretory cells the secretion may be fluid and poured forth at a free surface only, from which it is washed away by the currents of the circumambient medium. Of this type are the mu- cous cells which give rise to the slime which lubricates the skin of earthworms, or the similar cells which produce the mucus inside the human mouth cavity. On the other hand the secretion may take the form of fibres or another form of solid material and may be extruded from the entire surface of each cell so that the cell becomes in this way separated from its neighbours by masses of the secretion or, as it is now termed, the "intercellular substance." Cells of this kind are known as "connective tissue cells" and give rise to the various supporting tissues such as fibrous tissue, cartilage and bone.
The reproductive cells or "germ cells" are, as their name implies, cells which have the capacity of giving rise to a mass of cells out of which the body of a new individual is built up. They occur in many forms amongst the lowest grades of animals, i.e. the Protozoa, but amongst the higher animals, or Metazoa, they are remarkably con- stant in general characters, exhibiting practically the same forms throughout the whole range of animals from the sponges to the human race. They appear under two types, viz.: (l) a small motile male cell, which consists mainly of a head which is a con- densed nucleus, to which is added a vibratile flagellum termed the tail ; and (2) a much larger motionless female cell or egg, in the cyto- plasm of which there are deposits of food material known as yolk. Normally a male cell must unite with a female cell before develop- ment can be initiated, but the eggs of many animals can develop without this union and are then termed " parthenogenetic." In other cases where union with a male cell normally takes place eggs can be stimulated to develop without previous union with a male cell, and these cases are termed "artificial parthenogenesis."
Turning now to the consideration of the nucleus, we find that this is frequently invisible in the living cell, but when it can be made out it appears either as a clear vesicle (as in many eggs) or as a slightly more granular portion of the protoplasm (as in Amoeba). When, however, it is fixed and stained it exhibits a characteristic structure. It then appears as a vesicle containing a fluid termed the " nuclear
sap," and bounded by a membrane termed the " nuclear wall." The sap is traversed by a network of threads known as "linin fibres," and adhering to these fibres suggesting the arrangement of beads on a necklace are a number of granules termed "chromatin" be- cause they attract and retain the colouring matter of the staining fluid so strongly. In addition there are usually one or two rounded bodies termed " nucleoli," which also attract stain strongly, but these are apparently not composed of exactly the same chemical substances as the chromatin granules and it is possible by the use of certain stains to colour them differently from the tint assumed by the chromatin.
We have already seen that assimilation or the building-up of fresh protoplasm is impossible in the absence of a nucleus. This is proved by dividing a uninucleate amoeba into two, when it is seen that the non-nucleate piece, though capable of movement, cannot digest or assimilate food and, after a short period of activity, dies.
The division of the nucleus always precedes the division of the cell and this division is occasionally direct, i.e. by the process of simple constriction into two, or much more frequently indirect by the pro- cess termed " karyokinesis " or " mitosis." In this method the chroma- tin grains were believed to become aggregated into a continuous spirally-coiled thread termed the "spireme," which then became segmented into a definite number of segments called " chromosomes." The chromosomes always appear in the same number in the same species at every division of the nucleus. Meanwhile the nucleolus or nucleoli disappear, apparently dissolved in the cell-sap; and the nuclear membrane becomes dissolved so that the cell sap is mingled with the cytoplasm; and then a small body termed the "centro- some," lying at the side of the nucleus in the cytoplasm, becomes discernible. The centrosome can occasionally be demonstrated lying at the side of the resting nucleus, but when it becomes visible in the process of mitosis it is seen to be in process of division into two. The chromosomes at the same time appear to be split longitudinally, whilst from the daughter centrosomes as they move apart a series of filamentous rays are developed, some of which ("astral fibres") extend out to the periphery of the cell, whilst others (" mantle fibres ") become attached to the halves of the split chromosomes, one fibre from each centrosome going to a corresponding half of the chromosome. When the centrosomes have reached positions at opposite ends of the nucleus the mantle fibres proceeding from them have constituted the mitotic spindle: certain other fibres appear running directly from one centrosome to another in the axis of the spindle, which are termed " spindle fibres," whilst the split chromo- somes, under apparent tension of the mantle fibres, have assumed positions lying at right angles to the axis of the spindle, and con- stituted what is called the " equatorial plate."
By a continuance of the apparent tension of the mantle fibres the two halves of each chromosome are dragged apart from one another, and eventually one set of half-chromosomes becomes massed together in the vicinity of each pole of the spindle, that is, close to each daughter centrosome. Each of such sets of chromosomes constitutes a daughter nucleus. The chromosomes become longer and thinner and more entangled with one another until the initial stage of a linin network with adhering chromatin granules is re- established, and round this a new nuclear wall is formed. The stages leading to the segmentation of the spireme thread and the solution of the nuclear membranes are termed the " prophases of mitosis"; those leading to the establishment of the equatorial plafe are termed the " metaphase." The stage involving the separation of the halves of the chromosomes is termed the " anaphase," whilst the steps leading to the reconstitution of the daughter nuclei are termed the "telophases" of mitosis.
In the fertilization of the egg it is penetrated by the head of the spermatozoon which carries with it a segment of the tail termed the middle-piece. The middle-piece is distinguished by its diameter from the thinner more distal part of the tail and it includes a centro- some which has been derived from the centrosome active at the last division of the sperm mother cell. Sometimes the distal part of the tail is left behind at the surface of the egg sometimes it penetrates with the rest of the spermatozoon. In the latter case it remains passive in one cell of the embryo and is absorbed and is without .influence on the hereditary potencies of the embryo, since it is never distributed to any but the one cell. All the paternal in- fluence must be carried by the head, which is a condensed mass of chromatin: hence is derived the strong belief that not only is the nucleus the bearer of hereditary powers but that the chromatin is the element of the nucleus in which they are concentrated.
The spermatozoon head, once immersed in the egg, swells up and assumes the characteristic structure of a nucleus: it acquires nu- clear sap and a nuclear wall and its chromatin becomes resolved into the same number of chromosomes as are present in the egg nucleus. It is then known as the " male pro-nucleus," whilst the egg nucleus is termed the " female pro-nucleus." The centrosome derived from the middle piece gives rise to an enormous aster with radiating rays which is termed the " sperm-aster " ; the two nuclei approach each other, and the sperm-aster then becomes changed into a spindle, and the chromosomes of both nuclei are arranged side by side on the equatorial plate of this spindle: so that when the compound or " zygote " nucleus divides equal portions of maternal and paternal chromatin are distributed to the first two cells of the embryo and
