REPRODUCTIVE SYSTEM 



571 



which, by their ultimate divisions, give rise to spermatozoa. 

 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 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 yery 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.], which is first male and then female (protandrous) ; in some 

 species of the Teleostean genera Chrysopkrys and Serranns, 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 

 often been suggested that the original Vertebrate animals were 

 hermaphrodite. 



The quantity of yolk present in the egg varies very greatly in Verte- 

 brates, and its presence or absence exercises a profound influence upon 

 the processes of development. Following Hertwig, we may notice 

 that the presence of yolk has both a physiological and a morphological 

 effect. Physiologically, the presence of a store of nutriment enables the 

 developmental process to be carried on uninterruptedly, and the period 

 of independent life to be postponed until more or less complexity of 

 organisation has been attained. Morphologically, the yolk acts as a 

 check to the activity of the protoplasm, and by substituting an 

 embryonic mode of nutrition for that for which the adult organism 



