630 



VERTEBRATE LIFE AND ORGANIZATION 



Figure 30.12. Stages in the development of an egg, follicle and corpus luteum 

 in a nianinialian ovary. Successive stages are depicted clockwise, beginning at the 

 mesentery. Insets show the cellular structure of the successive stages. (Villee: Biology.) 



of the yolk sac and migrate to their final position in the gonad. Other 

 investigators maintain that the functional eggs do not come from these 

 primordial germ cells visible in the ovary at birth, but arise by new 

 proliferations from the germinal epithelium in the adult. 



As each oocyte develops, it becomes surrounded by other cells de- 

 rived from the germinal epithelium which form a spherical follicle 

 about it (Fig. 30.12). These cells proliferate and form a thick layer, 

 called the stratum granulosum, around the egg. A cavity, the antrum, 

 filled with liquid appears in the mass of follicle cells. The connective 

 tissue of the ovary forms a sheath, the theca, around the follicle. As 

 the follicle enlarges and its antrum becomes dilated with follicular 

 fluid, it is pushed near the surface of the ovary. It finally bursts and 

 releases the egg into the peritoneal cavity, whence it passes into the 

 oviduct. The release of the egg is known as ovulation. If the egg is fer- 

 tilized in the oviduct it will subsequently become embedded in the 

 lining of the uterus and begin development. 



The follicular cells remaining after the rupture of the follicle 

 multiply and increase in size, filling the cavity left by the follicle. Cells 

 from the theca grow in along with the granulosa cells and the two form 

 the corpus luteum. This yellowish structure, a solid mass of cells about 

 the size of a pea, projects from the surface of the ovary. If the egg is 

 fertilized the human corpus luteum persists for months, but if no fer- 



