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 this time Avas 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 i emaining 

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

 which are at first flat, then cubical, then cylindrical (fig. '21Q f). 

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



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



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

 ei, Egg; fz, follicular cells; fz\ follicular cells which envelop the ovum and constitute the 



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



folliculi) ; zp, zona pellucida. 



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. 21 1 A JQ. 



In const quence of a considerable increase of the fluid, the originally 

 solid follicle becomes converted finally into a large or small vesicle 

 (tig. 217 JB), which was discovered more than two hundred years ago ' 

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

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

 <>' /-(tafia it, follicle. Such a follicle (fig. 217 B} now consists of (1) an 

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



