348 



COMPARATIVE ANATOMY 



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 ow, while the smaller cells 

 form an investment of follicle round them, and serve as nutritive material. 

 The investing cells multiply, and in Mammals a cavity containing a fluid 

 is formed in the middle of each follicle (Fig. 276) : the main mass of the 

 follicular cells which enclose the ovum project, as the discus proligerus, into 

 the cavity of the follicle. When ripe, the ovum, surrounded by a vitelline 

 membrane, comes to the surface of the ovary and breaks through into 

 the abdominal cavity ; it then passes into the coelomic aperture of the 

 oviduct. 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 follicle in 

 which the ovum lay : this " wound " then closes up, and its contents undergo 

 fatty degeneration, giving rise to a body of 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. 

 These give rise to a series of seminal tubules (Fig. 300), containing larger and 

 smaller cells ; the former undergo division to form the sperm-cells or 



g 



FIG. 276. SECTION THROUGH A PORTION OF THE OVARY OF A MAMMAL, SHOWING 



THE MODE OF DEVELOPMENT OF THE GRAAFIAN FOLLICLES. 



KE, germinal epithelium, ingrowths from which extend into the stroma of the 

 ovary to form the ovarian tubes (PS) : the stroma is penetrated by vessels 

 (g.g) ; U, U, primitive ova ; S, cavity between the follicular epithelium 

 (tunica granulosa, M g) and the primitive ova ; Lf, liquor folliculi ; D, discus 

 proligerus ; Ei, ripe ovum, with its germinal vesicle (K) and germinal spot ; 

 Mp, zona pellucida, showing radiated structure ; Tf, theca folliculi. 



spermatozoa. The nucleus gives rise to the so-called "head" of the sperma- 

 tozoon, while the surrounding protoplasm becomes differentiated to form the 

 motile "tail," which serves as an organ of propulsion, the "neck" 

 (Mittelstiick) arising from the centrosome of the cell (p. 3). 



