cells of the individual. The difference in size is due simply to the concentration of the food yolk and most of the cytoplasm in one of the cells, the other three degenerating, being sacrificed to the production of an egg cell with the largest possible supply of nutritive substance in it.

Turning to the development of the sperm cell we find an exactly parallel series of stages, the end results, however, differing much in size. The mature The sperm cell. spermatozoon is an exceedingly minute. cell, consisting typically of a cylindrical or conical "head" containing a nucleus, a short cytoplasmic "middle piece," and a long vibratile "tail," an organ of locomotion differentiated out of the cytoplasm of the cell from which the spermatozoon is derived. The stages of multiplication, growth, and maturation are passed through in the development of the spermatozoon in the same order as in the egg development, save that the period of growth does not include the storage of food yolk in the primary spermatocyte, and the two divisions of the maturation stages are equal ones, resulting in the production of four cells of the same size, each of which develops into a complete spermatozoon. The accompanying diagrams of Fig. 9, taken from Boveri, illustrate clearly the homologies existing between the life histories of the two sorts of germ cells. The earlier stages of ovogonia and spermatogonia are indistinguishable from each other; later in the period of growth the increase in the size of the ovocyte marks it off from the minute spermatocyte, but this distinction is merely one due to non-living food material, and in no wise affects the fundamental identity of the two. In the maturation period the number of chromosomes in the nuclei of both egg and sperm is reduced one half-on the one hand, the ripe egg cell and three rudimentary egg cells (the polar

the two mature reproductive cells is enormous, the spermatozoa. The contrast in size which exists between

tids" are produced, which develop into four mature bodies) being formed; on the other, four equal "sperma

Growth period.

Maturation period.

FIG. 9.

Diagram illustrating the development of
the spermatozoon. (After Boveri.)

Diagram illustrating the development
of the egg. (After Boveri.)

spermatozoon in some cases containing less than 100 (Wilson), and in extreme cases less than 100 0 0 0 0 0 0 (Hertwig) of the volume of the egg cell.

Fertilization.

as may

A discussion of the method by which the reduction of the chromosomes in the germ nuclei is brought about may profitably be deferred until the essential features of fertilization have been examined. The phenomena of the fusion of egg and sperm can best be studied in some such form as the sea urchin, in which the egg is very small and, in some species, quite transparent. As fertilization takes place free in the sea water, the germinal cells being cast out from the parents, it is possible to collect the eggs and sperm separately from mature individuals and bring them together in small dishes of sea water, and at such times suit one's convenience. Then in the living egg much of the process may be followed under the microscope, and properly prepared sections of the eggs killed by reagents at the various stages enable conclusions to be drawn as to matters of minute detail. Fig. 10, A to F, presents a series of diagrams, taken from Boveri, illustrating the principal facts in the process of fertilization. In Fig. 10, A, the egg is represented with its clear nucleus in the centre, surrounded by the egg membrane. Clustered around the periphery are a number of spermatozoa endeavouring to find their way into the substance of the egg. On the right-hand side in the figure one has penetrated the membrane and is shown passing into the egg cytoplasm, which puts forth a small conical prominence to meet it. As soon as the head of one sperm enters the egg cytoplasm a new membrane is formed around the egg which effectually prevents the entrance of any others. The head and middle piece penetrate into the egg, the tail usually remaining imbedded in the membrane where it soon de

FIG. 10.-Diagrams illustrating the fertilization of the egg.

A, egg surrounded by spermatozoa; on the right, one has just penetrated the egg membranes and is entering the egg cytoplasm; egg nucleus in the centre. B, egg nucleus with chromatin reticulum on left; on right, the sperm nucleus preceded by its centrosome and attraction sphere. C, egg nucleus on the left, sperm nucleus on the right of the centre of the egg; stage immediately preceding the division of the centrosome. D, the centrosome has divided, the two attraction spheres separate to form the first cleavage spindle; the chromosomes of the egg and sperm nuclei clearly visible and indistinguishable (in the figure the egg chromosomes are black, the sperm chromosomes shaded). E, the first cleavage spindle, with splitting of chromosomes. F, completion of first cleavage; two-celled stage, each nucleus contains four chromosomes-two from the egg and two from the sperm. (After Boveri.)

generates. A few moments after the sperm has entered a system of radiations appears around the middle piece, which develops into an aster surrounding the centrosome of the sperm (Fig. 10, B). The sperm nucleus swells up and rapidly increases in size, its chromatin changing from the compact condition in which it is arranged in the sperm head to a reticulate condition (Fig. 10, C). The chromatin reticulum of the egg nucleus becomes also more clearly visible. Sperm aster and sperm nucleus now move in toward the egg nucleus, the aster usually preceding. As the nuclei approach the sperm nucleus increases still more in size until it becomes indistinguishable from the egg nucleus (Fig. 10, C). The chromatin network of each now breaks up into a number of chromosomes, one half of the number found in the somatic cells, and the nuclei come into contact, fusing together in some cases. In the sea urchin, Echinus, the number of chromosomes is eighteen, nine would therefore be found in the germ nuclei; for the sake of clearness and simplicity but two are represented in the diagram, those of the sperm nucleus being slightly shaded while those of the egg nucleus are black. The centrosome divides together with its aster Cleavage. (Fig. 10, D), the two daughter centrosomes move apart to opposite poles of the egg, and the typical amphaster of cell division is formed (Fig. 10, E), the nuclear membranes disappearing and the chromosomes being drawn together into the equatorial plate where each splits longitudinally. The halves are drawn by the mantle fibrils toward the opposite poles and the egg divides transversely into two cells (Fig. 10, F). This process of division is repeated continuously in each of the resulting generations of cells, and from the mass of cells thus formed develops the new organism. Each cell in the two-celled stage has received half of its