38 MAMMARY APPARATUS OF THE MAMMALIA 



muscles, but singularly rich in bloodvessels. 

 And now to think that we find in the primarv- 

 primordia exactly the same relations to the skin- 

 muscle and the vascular system ! It seems to 

 me justifiable, then, in view of these facts, 

 to compare the primary -primordia with these 

 brooding-spots, and indeed to interpret them 

 as vestiges of brooding organs. It must be 

 emphasized that in making this compai'ison, 

 we are dealing only with physiological analogy, 

 as it is in this way that we may conceive some- 

 thino; of the functions of those ora^ans. There 

 can of course be no question of homology 

 between such oi'gans in birds and mammals, 

 and their structural similarity is sufficiently 

 explained on functional grovinds. 



If you regard the primary-primordia as rudi- 

 mentary brooding organs, with which the 

 ancestors of the Mammalia hatched their eggs, 

 then a simple explanation of the origin of the 

 mammary apparatus presents itself. The trans- 

 formation of the pre-mammalian ancestors* into 

 Mammals brought with it the development of 

 the hairy coat and the skin glands appertaining 



* According to Cope and others, probably Synapsidan 

 reptiles of the stock of the triassic Cynodontia, 



