REPRODUCTIVE SYSTEM. 513 
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 
organs 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 4 
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 ovaare 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 AZyxine 
(g.v.), 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 ot 
Teleosteans, ¢.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 
often been suggested that the original Vertebrate animals were 
hermaphrodite. . 
33 
