Verfsbrata. 



351 



often attains an enormous size. The cells round the ovum secrete 

 a vitelline membrane, which is sometimes very thick. 

 The cells constitute, in all Vertebrata, a single layer round the 

 egg, and usually remain in this conditiou ; only in the Mammalia 

 do they divide, so that the young egg is surrounded by 

 several layers. Here a split appears later in the cellular mass 

 (Pig. 291, s), and gradually enlarges, so that the ripe Graafian 

 follicle of Mammalia looks like a hollow ball of cells, whilst the 

 ovum, surrounded by a proliferation of cells, projects into the cavity. 

 The ripe ova are shed into the body-cavity by the 

 bursting of the Graafian follicles. They vary in size in the different 

 Vertebrata ; they are smallest, 

 microscopic even, in the Mammalia, 

 largest in Birds and Selachians. 

 Where the ova are large, they 

 project from the surface of the 

 ovary, so that it looks very uneven ; 

 in Birds, it appears racemose, 

 whilst in Mammalia, on the con- 

 trary, it is usually a small, smooth, 

 roundish body. The ova gene- 

 rally escape by the Miillerian 

 ducts, a pair of long tubes 

 each opening at one end by a 

 ciliated funnel into the body-cavity 

 (usually near the ovary of the 

 same side), at the other end into 

 the c-loaca, or to the exterior by 

 a special aperture in the region 

 of the anus. (For the aberrant 

 relations of ovary and oviduct in 

 many Pisces, see that group.) 



The testes, of which there is also a pair, usually lie like the 

 ovaries, on the dorsal wall of the -body cavity {see the Mammalia for 

 the change of position which may occur during development) . They 

 consist of numerous coiled glandular tubes (seminal tubules), which 

 are closely packed, and in which the spermatozoa are produced by 

 the modification of the constituent cells. For the methods by which 

 the spermatozoa escape from the body, see above, pp. 349, 350. For 

 copulatory organs, see the different groups. 



Rudiments of tlie Miillerian ducts, varying in size, often occur in the male 

 (Selachii, Amphibia, Mammalia), just as vestiges of the mesonepliros (pai-ovarium) 

 and its duct (Gartner's duct), may sometimes he found in the female {e.g., in the 

 Ruminants). 



The majority of the Vertebrata are of separate sexes. Only in some species 

 of Teleostei are ova and spermatozoa formed in the same individual, i.e., there is 

 true herniaphroditism; both genital products develop in a common gland, 



Fig. 291. Section throug-h the ovary 

 of a Mammal ; diagrammatic, e epithelium 

 on the surface of the ovary, e' invaginated 

 portion of epithelium, g' young Graafian 

 / colHcle, g somewhat older do., s split, 

 03 egg, It, nucleus of egg. — Modified from 

 Wiedersheim. 



