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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 recognized 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

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FIG. 149.-Formation of the chromatins and tetrads in the spermatogenesis of Ascaris megalocephala. In A-C, the whole cell is shown; in D-H, only the nucleus. (After Brauer.)

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 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 degenerate, being sacrificed to the production of an egg cell with the largest possible supply of nutritive substance in it.

In the development of the sperm cell (Figs. 149, 150), we find an exactly parallel series of stages, the end results, however, differing much in size.

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B

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The mature spermatozoön 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 spermatozoön is derived. The stages of multiplication, growth, and maturation are passed through in the development of the spermatozoön 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 spermatozoön.

FIG. 150.-First (A-C) and second (D-H) maturation of the spermatocytes of Ascaris megalocephala. (After Brauer.)

The accompanying diagrams of Fig. 151, 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 size of an ovocyte marks it off from the minute spermatocyte, but this distinction is merely one due to nonliving food material, and

in nowise affects the fundamental identity of the two. 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 bodies) being formed; on the other, four equal "spermatids" are produced, which develop into four mature spermatozoa. The contrast in size which exists between the two mature reproductive cells is enormous, the spermatozoön in some cases containing less than 100000 (Wilson), and in extreme cases less than 10000000 (Hertwig) of the volume of the egg cell.

FIG. 151. At left, diagram illustrating the development of the spermatozoön; at right, diagram illustrating the development of the egg. (After Boveri.)

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. 153, A to F, presents a series of diagrams, taken from Boveri, illustrating the principal facts in the process of fertilization. In A, the egg is represented with its clear nucleus in the center, surrounded by the egg membrane. Clustered around the periphery are a number of spermatozoa endeavoring to find their way into the substance of the egg. On the righthand 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 degenerates. A few moments after the sperm has entered, a system of radiations.

appears around the middle piece FIG. 152.-Egg of the worm, Myzowhich develops into an aster surrounding the centrosome of the sperm (B). The sperm nucleus

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stoma glabrum, being fertilized by spermatozoön. (After Wheeler.)

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 (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 (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. As 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

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FIG. 153.-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 center. B. Egg nucleus with chromatin reticulum on the left; on the right, the sperm nucleus preceded by its centrosome and attraction sphere. C, Egg nucleus on the left, sperm nucleus on the right of the center of egg; aster fibrils preceding the fission of the centrosome. D. The centrosome has divided, the two attraction spheres separate to form the first cleavage of the spindle; the chromosomes of the egg and sperm nuclei clearly visible and distinguishable (in the figure the egg chromosomes are black, the egg sperm chromosomes shaded). E. First cleavage, showing spindle with splitting chromoF, Completion of first cleavage, two-celled stage, each nucleus showing four chromosomes, two from the egg and two from the sperm. (After Boveri.)

somes.

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