96 J. P. HILL. 



must assume to have taken place gradually and in correlation 

 with the longer retention of the egg in the oviduct, the 

 elaboration of the uterine portion of the same as an actively- 

 secretory organ, and the evolution of the mammary apparatus. 

 The Monotremes thus render concrete to us one of the first 

 great steps in mammalian evolution so far as developmental 

 processes are concerned, viz. the substitution for intra-ovular 

 yolk of nutrient material furnished directly by the mother to 

 the developing egg or embryo. We see in them the begin, 

 nings of that process of substitution of uterine for ovarian 

 nutriment which reaches its culmination in the Eutheria with 

 their mici'oscopic yolk-poor ova and long iutra-uterine period 

 of development. The Marsupials show us in Dasyurus an 

 interesting intervening stage so far as the ovum is concerned, 

 in that this, though greatly reduced as compared with that 

 of the Monotreme, still retains somewhat of its old tendencies 

 and elaborates more yolk-material than it can conveniently 

 utilise, with the result that it has to eliminate the surplus 

 before cleavage begins. But as concerns their utilisation of 

 intra-uterine nutriment, they have specialised along their 

 own lines, and instead of exhausting the possibilities implied 

 by the presence of that, they have extensively elaborated 

 the mammary apparatus for the nutrition of the young, born 

 in a relatively immature state, after a short period of intra- 

 uterine life (cf. Wilson and Hill ['97, p. 580]). 



In view of the fact that the young Monotreme enjoys three 

 developmental periods, viz. intra-uterine, incubatory, and. 

 lactatory, the question might be worthy of consideration 

 whether it may not be that the Marsupial has merged the 

 incubatory period in the lactatory, the Eutherian the same in 

 the intra-uterine. 



2. The Early Development of the Metatheria and 



Eutheria. 



It will have become evident from the foregoing that the 

 Metatherian mode of early development is to be regarded as 



