300 COMPARATIVE ANATOMY. 



say which will give rise to spermatozoa, and which to ova. In 

 the course of further development, this differentiation takes place, 

 and the germinal epithelium comes into relation with the meso- 

 nephros, as already mentioned. The final result is the formation 

 of a male or a female generative gland, i.e. a testis or an ovary. 



The mode of development of the ova and spermatozoa is briefly as follows : 

 Ova. The cells of the germinal epithelium grow inwards amongst the 

 stroma of the ovary in the form of clustered masses : some of these increase 

 in size more than the others, and give rise to the primitive ova, while the 

 smaller cells form an investment or follicle round them, and serve as a 

 nutritive material. The investing cells multiply, and in Mammals a cavity 

 containing a fluid is formed in the middle of each follicle (Graafian follicle) 

 (see Fig. 254) : the main mass of the follicular' cells which enclose the ovum 

 project, as the discus proligerus (Fig. 254, Z)), into the cavity of the 

 follicle. When ripe, the ovum, surrounded by its vitelline membrane, comes 

 to the surface of the ovary and breaks through its walls into the abdominal 

 cavity ; it then passes into the Fallopian tube by means of the cilia on the 

 fimbriated aperture of the latter. A certain amount of blood is poured out 

 through the broken ends of the vessels in the stroma of the ovary into the 

 cavity of the Graafian follicle in which the ovum lay : this " wound " then 

 closes up, and its contained blood undergoes fatty degeneration, and gives rise 

 to a body of a yellow colour, known as the corpus luteum. 



Spermatozoa. As in the case of the female, primitive germinal cells 

 can be at first distinguished in the development of the male generative elements. 

 The nucleus of the larger of these gives rise to the main part of the so-called 

 "head" of the spermatozoon, while the surrounding protoplasm becomes 

 differentiated to form the motile " tail," which serves as an organ of propulsion. 

 Either each of the primitive germinal cells forms one spermatozoon only, 

 or a division of its nucleus into several portions takes place, and the cell 

 divides up into several spermatozoa. But there is no important difference 

 between these two modes of development, for in both cases we have to do with 

 a simple cell-division. Thus each spermatozoon is really the mor- 

 phological equivalent of an ovum, so that an absolutely similar 

 and equal portion from either parent is concerned in the 

 production of the embryo. 1 



In order to understand the general relations of the urinogenital 

 organs as a whole, we must now consider briefly the canals which 

 carry off the generative products. 



In Cyclostomi and a very few other Fishes the generative 

 products are simply shed from the ovary or testis into the body- 

 cavity, whence they pass to the exterior by means of the abdo- 

 minal pores (see p. 265). This is probably to be looked upon 

 as the most primitive condition. 



In all other Vertebrates except bony Fishes, a canal, called the 

 Mullerian duct, is formed in the female. This either becomes 

 split off from the segrnental duct (Elasmobranchs, Amphibia), or 

 arises independently as an involution of the peritoneum near the 

 latter (Amniota). 2 



1 It must be remembered that the process of fertilisation has to do essentially 

 with the fusion of the nuclei only of the male and female cells (Weismann and 

 Van Beneden). (Comp. pp. 3 and 4.) 



3 According to Mikalovics, the Miillerian duct arises in the Lizard, Duck, and Chick 

 as a solid mesodermal rod, arid thus has originally no connection with the ccelome. 



