OOGEXESIS. 



293 



ovary are chiefly oogonia. The vesicular follicles contain oocytes which 

 may be recognized by their great size (about 200 // in diameter). As 

 seen in Fig. 335, the nucleus is large and vesicular [and is often called the 

 germinative vesicle]. It contains a nucleolus [germinative spot] which 

 in fresh liquor folliculi exhibits amoeboid movements. The nucleus has 

 a distinct membrane; usually if is near the center of the cell, but it may 

 migrate to the periphery. The central part of the protoplasm contains 

 coarse granules of yolk derived from the follicular cells; it is surrounded 

 by a finely granular zone, and this is followed by a very narrow layer 

 free from granules. The protoplasm of oocytes may contain a "yolk 

 nucleus," a structure formed by the centrosome and archoplasm or idio- 

 zome. Yolk nuclei are not found in mature ova. The oocytes probably 

 possess no distinct cell wall. They are surrounded 

 by a broad, clear, radially striated band, the zona 

 pellucida. The striations are said to be canals 

 containing processes of the follicular cells. It is 

 still doubtful whether the zona is a product of the 

 oocytes or of the follicle. The egg cell may become 

 separated from it by a narrow perivitelline space as 

 shown in Fig. 335. 



When the primary oocyte divides into the 

 secondary oocytes the nuclear material is equally 

 distributed between them. One of them, how- 

 ever, receives nearly all the protoplasm; conse- 

 quently the other is a small cell and is known as 

 the first polar globule. In becoming a mature ovum 

 the secondary oocyte divides for the second and 

 last time, thus giving rise to the ovum and second 



polar globule. The first polar globule may divide in two. Thus the pri- 

 mary oocyte produces a mature ovum, and three polar globules which 

 from their lack of protoplasm are generally functionless. As they occur in 

 the mouse they are shown beneath the zona pellucida in Fig. 336. It 

 is unknown when the polar globules are formed in man, whether in the 

 ovary before ovulation, or later. In the mouse one forms in the ovary 

 and in the other in the uterine tube. 



Fertilization. The ovum passes from the peritoneal cavity into the 

 fimbriated end of the uterine tube, and in the upper part of the tube it may be 

 fertilized. The process in man is unknown, but from observations in other ani- 

 mals it is probable that several spermatozoa enter the zona pellucida, and that 

 only one passes into the protoplasm of the ovum. It loses its tail piece as it 

 enters. The head is resolved into twelve (?) chromosomes which become ar- 

 ranged beside the twelve (?) in the nucleus of the ovum. The centrosomes 



FIG. 336. OVUM OP WHITS 

 MOUSE, SURROUNDED 

 BY ZONA PELLUCIDA. 



Above the ovum are two 

 polar globules; within 

 it are two nuclei, one 

 belonging to the ovum, 

 the other being de- 

 rived from the head of 

 the spermatozoon. X 

 500. (After Sobotta, 

 from Minot's " Em 

 bryology.") 



