quantity, it was at the critical phase, that au allantoie placenta was 

 either formed, or first came into operation^). 



Moreover, it was established, that the evolution of mammary glands 

 and the initiation of lactation had been the primary factors in reducing 

 the yolk in the egg of ancestral mammals. This reduction, due to the 

 mammae, could, however, be carried no further back than the critical 

 period, i. e., than the epoch, at which the embryo, being first com- 

 plete in all its parts, could, and must, provide for its own nutrition. 

 The mammary organs could not, and cannot, begin to function prior 

 to that time. 



If, as has happened, a yolk-sac placenta, or trophoblast, be de- 

 veloped, it, as a structure forming a part of the asexual generation 

 or phorozoon, can only function until the critical period, when it 

 must begin its degeneration, for its life-span is reached. 



The combined workings of these two factors, mammary and tro- 

 phoblastic nutritions, from opposite points, the one beginning at the 

 soonest at the critical period, the other ending at the latest at this 

 important epoch, have brought it about, that, where no allantoic pla- 

 centa is developed, there uterine life is only possible until the critical 

 period, when the young must, perforce, be born. This is probably the 

 case in all aplacental Marsupials, and it certainly holds in Hypsi- 

 prymnus, Didelphys, Trichosurus and M a c r o p u s , as either 

 demonstrable, or already established. 



Once the birth was fixed to take place at the critical period in 

 ancestral Metatheria and Eutheria, an alteration of this arrangement 

 could only be brought to pass by the evolution of an allantoic pla- 

 centa. If this happened, the birth could be postponed to a later time, 

 for the nutrition of the foetus beyond the critical period ^) was pro- 

 vided for. 



Before an allantoic placenta had been, so to speak, invented, the 

 coincidence of birth- and critical periods had led ^^to deeply - reaching 

 etiects on the ovulation. 



Prior to all these changes, evolution of mammae, trophoblast, etc., 

 ovulation may have occurred at regular periodic intervals, but of this 

 there is as yet no evidence available in the developmental history of 



1) More recent work has demonstrated the correctness of the latter 

 alternative. 



2) The critical period is one of starvation rather than, as Houssay 

 suggests, of asphyxation (compare F. Houssay, Le rappel ontogenetique 

 d'une metamorphose chez les Vertebres, Anat. Anz., Bd. 13, p. 33 — 39). 



