REPRODUCTIVE SYSTEM. 493 



the cortex from the Wolffian body, or even from the most anterior part 

 of the germinal epithelium. On the other hand, some investigators 

 derive the medulla from metamorphosed cortical cells. There is much 

 evidence (morphological and physiological) that the suprarenals of 

 Elasmobranchs correspond to the medullary part in Mammals, while 

 the interrenals of Elasmobranchs and the suprarenals of Teleosts and 

 Ganoids correspond to the cortical portion in Mammals. 



^Yith regard to function, there is even more uncertainty. The supra- 

 renal bodies are relatively very large in embryonic life, but fail to 

 maintain their primitively rapid rate of growth. It has been suggested 

 that they assist in breaking down or disposing of waste pigment. 



Reproductive system. - - The ovaries and testes are 

 developed from a ridge formed by a part of the epithelium 

 lining the abdominal cavity, this ridge constituting the 

 so-called germinal epithelium. 



In the male the proliferating germinal epithelium is 

 divided by embryonic connective tissue into numerous 

 follicles. The cells of the follicles form seminal mother 

 cells, which, by their ultimate divisions, give rise to sper- 

 matozoa. From the mesonephros, tubules grow out to the 

 embryonic testes ; these form the collecting tubes of the 

 organ and open into the Wolffian duct, the vas deferens 

 of the adult. 



In the female the ovary is similarly divided up into 

 follicles. In this case, however, differentiation sets in 

 among the originally equivalent cells of the follicle. One 

 cell in each follicle is more successful than its neighbours, 

 which are sacrificed to form an envelope of follicular cells 

 around the single large ovum cell. The ova are usually 

 shed into the body cavity, and pass thence to the exterior 

 by the Miillerian ducts or oviducts. 



In many cases, between the follicular cells and the ovum there is a 

 membrane, the zona radiata, which is traversed by fine pores, and, in 

 consequence, has a striated appearance ; other egg membranes, more or 

 less transitory in nature, also occur. In the lower Vertebrates the layer 

 of follicle cells is single, but in Mammals (except in Monotremes) it is 

 multiple, and a quantity of clear fluid accumulates between the cells 

 and the ovum. The whole forms a " Graafian follicle," which bursts 

 when the ovum is liberated. 



Before fertilisation takes place, the ovum undergoes a process of 

 maturation, during which extrusion of polar bodies typically occurs ; 

 the technical difficulties in the way of the definite observation of this 

 fact are, however, often very great. The ova are fertilised outside the 

 body in Cyclostomata, Ganoids, Teleosteans, Dipnoi, and tailless 

 Amphibians ; internally in the other Vertebrates. 



