THE ORGANS OF THE MIDDLE GERM-LAYER. 379 



or even hours. The discharge takes place by the rupture of the 

 connective-tissue envelope, which causes the eggs to escape into the 

 body-cavity, as in the Fishes and most of the Amphibia. After the 

 elimination, the ovary, which up to tin's time was extraordinarily 

 large and took up most of the space in the body-cavity, shrivels into 

 a very small cord and now encloses only the young germs of ova, 

 part of which are destined to mature during the next year. 



The formation of the follicle takes place in a somewhat different 

 way in Mammals. The follicle originally contains, as in the remaining 

 Vertebrates, only a single egg and a single layer of follicular cells, 

 which are at first flat, then cubical, then cylindrical (fig. 216 /). 

 For a long time these cells envelop the egg as a single layer, but 



^fk 



Fig. 217 A and B. Two stages in the development of the Graafian follicle. A with the f ollicular 



fluid beginning to be formed ; B with a greater accumulation of it. 

 ti, Egg ; fz, f ollicular cells ; fz l , follicular cells which envelop the ovum and constitute th 



discus proligerus; ff, follicular fluid (liquor folliculi); fk, follicular capsule (theca. 



folliculi) ; zp, zona pelluuida. 



they then grow, undergo division, and are converted into a thick 

 envelope of many layers. But the difference from the course of 

 development described above becomes still greater, owing to the fact 

 that a fluid, the liquor folliculi, is secreted by the proliferated 

 follicular cells, and collects in a small cavity at the side of the egg 

 (fig. 217 Aff). 



In consequence of a considerable increase of the fluid, the originally 

 solid follicle becomes converted finally into a large or small vesicle 

 (fig. 217 B), which was discovered more than two hundred years aga 

 by the Hollander REGNIER DE GRAAF and was held to be the 

 human ovum. The structure has also been named after him the 

 trraafian follicle. Such a follicle (fig. 217 B) now consists of (1) an 

 outer connective-tissue, vascular envelope (fk), the theca folliculi ; 



