7O OVA OF MAMMALS CHAP. 
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 being, 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 le in the 
—— 
erraa aaa oa D0DG 
bl.v 
J SR 
ZINA 
Fic. 44.—Part of a sagittal section of an ovary of a child just born. 3/.v, Blood-vessels ; 
foll, strings and groups of cells derived from the germinal epithelium becoming 
developed into follicles; g.ep, germinal epithelium; iz, ingrowing cord of cells 
from the germinal epithelium ; pr.ov, primitive ova. (From Hertwig, after 
Waldeyer. ) 
abdominal cavity behind the kidneys. In the immense majority 
of the Mammalia the ova which are produced within the 
ovaries are of minute size; those of even the colossal Rorqual 
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 living 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 embryo, and partly of portions of the lining 
membranes of the uterus of the mother. The ova of the 
