466 STRUCTURE OF VERTEBRATA. 



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 mesonephric duct, the vas deferens 

 of the adult. This is the most typical Vertebrate condition, 

 but is not universal (see Table, p. 465). 



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. 



Hermaphroditism occurs as a normal state in Tunicata, most of which 

 are first functionally female and then male (protogynous) ; in Myxine 

 {q.v.) t which is first male and then female (protandrous) ; in some 

 species of the Teleostean genera Chrysophrys and Serranus, of which 

 the latter is regularly self-fertilising ; and in a solitary Batrachian. It 

 occurs casually in some Selachians, in the sturgeon, in about a score of 

 Teleosteans, e.g. cod, in various Amphibians, and more rarely in 

 Amniota. There are also embryological facts which suggest that the 

 embryos of higher Vertebrates pass through a state of hermaphroditism 

 before the unisexual condition is reached. On these grounds it has 



