ORIGIN IN PLACENTALS 133 



primary-primordia and milk-streaks and the 

 close affinity subsisting between the nipples and 

 milk glands of the Marsupials and the Placen- 

 tals, we can now clearly picture to ourselves 

 the phylogenetic processes which led to the 

 origin of the mammary apparatus in the 

 Placentalia. We have to start with forms 

 which, like the primitive Didelphyidae, pos- 

 sessed simple e version nipples and milk glands 

 developed on the basis of the former brooding 

 organs. These nipples and milk glands could 

 be directh' taken over bj^ the first Placentals, 

 whilst owing to the prolongation of the intra- 

 uterine period of development, formations like 

 the incubatorium or marsupium were not 

 needed. Only trifling changes took place, 

 such as the entirely retrograde development 

 of the mammary hairs, and variations in the 

 details of nipple formation, and finally the 

 establishment of a nipple-row arrangement. 



The entire course of the phylogeny seems 

 perfectly plain and straightforward, and might, 

 in consequence of its simplicity, have been 

 recognized long ago ! But here again the 

 unfortunate supposition that the marsupium 

 or the non-existeut mammary pouches were 



