Fig. 1.—Human ovum, from a mature follicle, a sphere of about 0.2 mm. diameter. × 25. (Kollmann.)

Fig. 2.—Segmentation of mammalian ovum (bat). (After E. von Beneden.) Two blastomeres, each with a nucleus, shown in lighter color. The dark bodies are yolk-granules.

Fig. 3.—Segmentation of mammalian ovum. Four blastomeres. (After E. von Beneden.)

The entire vertebrate body is the product of developmental changes taking place after fertilization in a single primitive CELL, the EGG or OVUM (Fig. 1).

In structure the ovum corresponds to other animal cells. On account of their special significance during development the different component parts of the egg-cell have received special distinctive names. The cell-body is known as the vitellus or yolk. It is composed of two substances, the protoplasm or formative yolk and the deuteroplasm or nutritive yolk, which vary in their relative proportions in the ova of different animals.

The protoplasm represents the material from which in the course of development the cells forming the body of the individual are derived, while the deuteroplasm serves for the nutrition of the ovum during the earliest stages of development.

The nucleus of the egg-cell is distinguished as the germinal vesicle, and its nucleolus as the germinal spot.

The cell-body or vitellus is surrounded by a condensed portion of the cell contents to which the name of the vitelline membrane has been applied, which in turn is enclosed by a transparent and elastic cover, the zona pellucida, presenting a radially striated appearance.

The ovum is contained in the cortical portion of the ovary, enclosed in the Graafian follicle, a vesicle 4-8 mm. in diameter, whose fibrous walls are lined by several layers of epithelial cells, which surround the ovum, forming the discus proligerus.

After impregnation the egg-cell, by a process of repeated division or cleavage, undergoes segmentation, the cell-body being divided successively into two, four, eight, sixteen, thirty-two, etc., cells, called blastomeres (Figs. 2 and 3). The mass of cells finally resulting from this process of segmentation forms the ground work of the future body. A vertebrate ovum in this stage of complete segmentation is called the morula from its resemblance to a mulberry (Fig. 4).

After segmentation is completed a cavity filled with fluid and surrounded by the developing cells is gradually formed in the interior of the mass. This cavity is known as the segmentation-cavity. The egg is now called the blastula, blastosphere or blastodermic vesicle and the cellular membrane enclosing the segmentation-cavity forms the germinal membrane or blastoderm (Figs. 5 and 6). The cells of the blastoderm become aggregated at one point on the circumference of the vesicle (dorsal pole of blastosphere) forming, when viewed from above, a thickened biscuit or disk-shaped opaque area. This is known as the germinal area, or primitive blastoderm or embryonic shield (Figs. 7 and 12).

Fig. 4.—Ovum of rabbit, from terminal portion of oviduct. The zona pellucida appears thickened, and contains many spermatozoa which failed to penetrate the ovum. (After Bischoff.)

Fig. 5.—Blastodermic vesicle of rabbit. (After E. von Beneden.)

Fig. 6.—Blastodermic vesicle of Triton tæniatus. (Hertwig.)

Fig. 7.—Embryonic area of rabbit embryo. (Heisler, after E. von Beneden.) The primitive streak beginning in the cell-proliferation known as the “node of Hensen.”

This is the first indication of the coming division of the entire egg-cell into the embryo proper and the vitelline or yolk-sac (Figs. 8 and 9). The entire future individual develops from the cells of the germinal area. This area comprises both the embryo proper and the region immediately surrounding it.

Fig. 8.—Blastodermic vesicle of mammal. (E. von Beneden.) The layer of cells lining the interior of the vesicle next to the zona pellucida forms Rauber’s “Deckschichte” or prochorion. This is not the true ectoderm, since it does not participate in the formation of the embryo, which is entirely derived from the cells of the germinal area.

Fig. 9.—Human embryo with yolk-sac, amnion, and belly-stalk of fifteen to eighteen days. (Heisler, after Coste.)

The remainder of the ovum, serving temporary purposes of nutrition and respiration, gradually becomes absorbed and disappears.

Fig. 10.—Embryonal area of sheep, composed of ectoderm and entoderm. (After Bonnet.)

Fig.