

DEVELOPMENT OF THE OVARIES AND OVA. 201 



The various and in part very artificial classifications that have 

 been sought to be established in ova are consequently resolved 

 in a very simple manner. 



It is more difficult to institute a comparison between the 

 ovaries of different animals. In the lowest classes these appear 

 to be reduced to their simplest yet most essential element, the 

 egg cell, and we even find in many Vermes and Ccelenterata, 

 in lieu of special organs, merely a few regions of the body wall 

 coated with germinal epithelium, without any specially or- 

 ganized vascularized support, and the cells of this germinal 

 epithelium increase without more ado into ova. Echinodermata 

 Mollusca, and by far the greatest number of the Arthropoda, 

 exhibit, on the other hand, special organs constructed upon the 

 type of a tubular or acinous gland. And we further find in 

 Mollusks and most Arthropods that arrangement of the 

 Graafian follicles which is persistent in, and I might almost 

 say which constitutes the characteristic feature of, the Verte- 

 brata. In accordance with this the primordial ovum becomes 

 enclosed by the growth of secondary structures in a peculiar 

 cavity that is completely surrounded by a vascular stroma. 

 The contrast appears to be worthy of notice that occurs in this 

 respect between the higher Vertebrata and the lower, as the 

 Batrachians and Osseous Fishes, and most of the Invertebrata. 

 In the former a portion at least of the ovarial epithelium con- 

 stantly remains freely exposed, whilst amongst the latter the 

 whole of the epithelium is completely enclosed by the vascu- 

 lar stroma. We must conceive this enclosure amongst the 

 Amphibia and Osseous Fishes so to occur that the germinal 

 epithelium, which in all Vertebrata is expanded in a flattened 

 manner, as well as the epithelium of the tube, becomes, as 

 development proceeds, everywhere surrounded by a growth of 

 vascularized stroma. In most Osseous Fishes this process is 

 completed in immediate continuity with the Fallopian tube, 

 the ovarium then appearing as the csecal and expanded ex- 

 tremity of the latter. In the higher Vertebrata this total 

 imbedding does not occur, being deficient at the ostium ab- 

 dominale of the Fallopian tube. 



The entire rudiment of the ovary is constructed precisely 

 upon the type of the proper, i.e. of the epithelial, glands. We 



