348 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



layer of epithelial cells, the whole forming a Graafian follicle, the 

 foUicle cells supplying nourishment to the contained ovum. In the 

 higher vertebrates there is a great increase in the number of follicle 

 cells, which become arranged in several layers. Then in mammals 

 (fig. 372) a split arises in the follicle, the cavity becoming filled with 

 a follicular liquor, while the ovum, surrounded by several layers of 

 cells, adheres to one side of the cavity, this part being called the 

 discus proligerus. 



When the eggs have attained their full size and the proper time 

 has arrived, some of the follicles migrate to the surface of the ovary, 

 then the follicles rupture and the contained ova escape into the 

 coelom. Their history from this point will be outlined in connexion 



with the genital ducts. Each ruptured 

 follicle (at least in elasmobranchs, amphib- 

 ians, and amniotes leaves a scar on the sur- 

 face of the ovary — the corpus luteum — 

 characterized by the presence of peculiar 

 ('lutein') cells. 



Testes.^ — In the gonads of the male 



(testes) there is a somewhat similar insink- 



ing of the primordial ova and epithelial cells 



into the stroma of the genital ridge, but, in- 



„.,,,., stead of breaking up into separate follicles. 



Fig. 372. — Graafian folhcle o r- r- 



of cat. c, connective-tissue each sexual cord develops a lumen and 

 ^:SeLl;f?vuT;r"SS tecomes converted into a seminiferous 

 granulosum (follicle cells); z, tubulc, in the walls of which both the 



zona pellucida. • i t i n i i • t i 



epithelial cells and the primordial ova are 

 recognizable, as well as a third kind of cell, called Sertoli's cell, con- 

 cerning which accounts are somewhat at variance, some regarding 

 them as derivatives of the epithelial cells, others as coming from the 

 primitive germ cells. They play no part in the actual formation of 

 the spermatozoa, but act rather as nutritive or 'nurse cells' for the 

 developing spermatozoa. For the differentiation of the germ cells 

 into spermatozoa reference must be made to the text-books of em- 

 bryology and histology. In most vertebrates the testes continue in 

 the position where they first appear, but in most mammals they 

 eventually descend to a position outside of the body cavity and are 

 enclosed in a special pouch, the scrotum. This descent of the testes 

 is described in connexion with the reproductive organs of the mam- 

 mals, below. 



