certain stages preliminary to their union, which are essentially alike. The animal egg is a large, more or less spherical cell, enveloped usually by certain membranes, containing a large The vast bulk of the egg

The egg cell.

nucleus and cytoplasm. cell, however, is made up of inert food material in the form of yolk granules, which are stored up in it as nourishment for the developing embryo. The nucleus, or germinal vesicle, is large, and contains a network of chromatin together with one or more conspicuous nucleoli. There are three periods usually recognised in the development of the egg cell, viz.: 1. The period of multiplication; 2, the period of growth; and, 3, the period of maturation. The first period is characterized by a continued series of divisions of the primitive reproductive cell and its descendants, which produces a large number of "ovogonia." Succeeding this is a period of growth in which the ovogonia increase greatly in size, mainly through the production and storing up of food yolk. At the close of this period the germ cell, now termed a "primary ovocyte," enters upon the maturation period, in which it undergoes two divisions in rapid. succession, by means of which two minute

Maturation.

cells, the polar bodies, are cut off from the egg. Through these two divisions the number of chromosomes in the egg nucleus is reduced to one half that which is found in the other cells of the body. The first polar body also usually divides, and thus, at the close of the period of maturation, four cells result, one large mature egg cell, ready for the fertilization which initiates the development of the embryo, and three minute polar bodies, which are to be regarded simply as rudimentary eggs. The nuclei of these four cells are exactly alike in that they all contain the same number of chromosomes—i. e., one half the number in the somatic

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 100000 (Wilson), and in extreme cases less than 100000000 (Hertwig) of the volume of the egg cell.

Fertilization.

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 as may 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.)