70 



OVA OF MAMMALS 



and on the left being different. The lungs of mammals agree 

 with those of the lower reptiles in being freely suspended within 

 their coelomic cavity, and in not l)eing, as in birds, crocodiles, 

 and the Varanidae among lizards, tied down to the dorsal surface 

 of that cavity by a sheet of peritoneum covering them. 



The Gonads (Ovaries and Testes).— The ovary in the 

 Mammalia is always paired ; there is never a partial or com- 

 plete abortion of one gonad as in birds — except of course in 

 pathological cases. The ovaries are small, and lie in the 



bl.V 



./clL 



Fig. 44.— Part of a sagittal section of an orary of a child ju.st horn. U.v, Blood-vessels ; 

 foil, strings and groups of cells derived from the germinal epithelium becoming 

 developed into follicles ; g.ep, germinal epithelium ; in, ingrowing cord of cells 

 from the germinal epithelinm ; jn-.ov, primitive ova. (From Hei-twig, after 

 Waldeyer.) 



abdominal cavity behind the kidneys. In the immense majority 

 of the Mammalia the ova which are jiroduced within the 

 ovaries are of minute size ; those of even the colossal Eorqual 

 are, so far as we know, not markedly larger than the ova of a 

 Mouse. The smallness of size of these reproductive elements 

 implies necessarily an absence of much nutritive yolk ; and as a 

 consequence the developing embryo, since it is not hatched in an 

 early stage as a free lining larva, has to be nourished by the 

 mother, to whose tissues it is attached through the intermediary 

 of the placenta, a structure partly composed of foetal structures 

 derived from the emljryo, and partly of portions of the lining 

 membranes of the uterus of the mother. The ova of the 



