ing the embryo in the early stages of its development only, whereas in the egg 

 of the bird there is sufficient to supply the chick with nutriment throughout 

 the whole period of incubation. The nutritive yolk not only varies in amount, 

 but in its mode of distribution within the egg; thus, in some animals it is almost 

 uniformly distributed throughout the cytoplasm; in some it is centrally placed and 

 is surrounded by the cytoplasm; in others it is accumulated at the lower pole of the 

 ovum, while the cytoplasm occupies the upper pole. A centrosome and centriole 

 are present and lie in the immediate neighborhood of the nucleus. 



Germinal Vesicle. The germinal vesicle or nucleus is a large spherical body 

 which at first occupies a nearly central position, but becomes eccentric as the growth 

 of the ovum proceeds. Its structure is that of an ordinary cell-nucleus, viz., it 

 consists of a reticulum or karyomitome, the meshes of which are filled with 

 karyoplasm, while connected with, or imbedded in, the reticulum are a number 

 of chromatin masses or chromosomes, which may present the appearance of a 

 skein or may assume the form of rods or loops. The nucleus is enclosed by a 

 delicate nuclear membrane, and contains in its interior a well-defined nucleolus 

 or germinal spot. 



Coverings of the Ovum. The zona striata or zona pellucida (Fig. 3) is a thick 

 membrane, which, under the higher powers of the microscope, is seen to be radially 

 striated. It persists for some time after fertilization has occurred, and may serve 

 for protection during the earlier stages of segmentation. It is not yet determined 

 whether the zona striata is a product of the cytoplasm of the ovum or of the cells 

 of the corona radiata, or both. 



The corona radiata (Fig. 3) consists or two or three strata of cells; they are 

 derived from the cells of the follicle, and adhere to the outer surface of the zona 

 striata when the ovum is set free from the follicle; the cells are radially arranged 

 around the zona, those of the innermost layer being columnar in shape. The 

 cells of the corona radiata soon disappear; in some animals they secrete, or 

 are replaced by, a layer of adhesive protein, which may assist in protecting and 

 nourishing the ovum. 



The phenomena attending the discharge of the ova from the follicles belong 

 more to the ordinary functions of the ovary than to the general subject of embry- 

 ology, and are therefore described with the anatomy of the ovaries. 1 



Maturation of the Ovum. Before an ovum can be fertilized it must undergo 

 a process of maturation or ripening. This takes place previous to or immediately 

 after its escape from the follicle, and consists essentially of an unequal subdivision 

 of the ovum (Fig. 4) first into two and then into four cells. Three of the four 

 cells are small, incapable of further development, and are termed polar bodies or 

 polocytes, while the fourth is large, and constitutes the mature ovum. The process 

 of maturation has not been observed in the human ovum, but has been carefully 

 studied in the ova of some of the lower animals, to which the following description 

 applies. 



It was pointed out on page 37 that the number of chromosomes found in the 

 nucleus is constant for all the cells in an animal of any given species, and that in 

 man the number is probably twenty-four. This applies not only to the somatic 

 cells but to the primitive ova and their descendants. For the purpose of illustrating 

 the process of maturation a species may be taken in which the number of nuclear 

 chromosomes is four (Fig. 5). If an ovum from such be observed at the beginning 

 of the maturation process it will be seen that the number of its chromosomes is 

 apparently reduced to two. In reality, however, the number is doubled, since 

 each chromosome consists of four granules grouped to form a tetrad. During the 

 metaphase (see page 37) each tetrad divides into two dyads, which are equally 



1 See description of the ovary on a future page. 



