REPRODUCTIVE SYSTEM. 463 



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 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 



Myxiiie (q. v.), which is first male 



and then female (protandrous) ; in 



some species of the Teleostean 



genera Chrysophrys and Serranits^ 



of which the latter is regularly 



Fig. I49.-Mammalian Ovum. self-fertilising ; and in a solitary 



(After Hfrtwig ) Eatrachian. It occurs casually m 



some Selachians, in the sturgeon, 



m., Ovum; /, follicular capsule; jn about a score of Teleosteans, 



/=., follicle cells; /c., follicle cells , ■ .^^rious Amphibians, 



forming discus proligerus ; /.I., cavity .5 ' ' , K ■ ^ 



occupied by liquor follicuH. and more rarely in Amniota. 



There are also embryological facts 

 which suggest that the embrj'OS 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 Verte- 

 brate animals were hermaphrodite. 



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

 Vertebrates, 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. Again, morphologically, 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 is 



