The Birth-period of Trichosurus vulpecula. 39 



result should not be under-estimated. "When the coincidence of birth- 

 and critical periods can thus be established in non-placental marsu- 

 pials, so widely separated as Didelphys, Trichosurus and JBypsiprymnus^ 

 the conclusion appears warranted, that in all non-placental marsupials 

 the same coincidence of birth- and critical periods must obtain. 



It is, at any rate at the present day, a necessity, from which no 

 escape is possible, that in non-placental marsupials the birth should 

 take place at the critical period. Prior to this it cannot happen, if 

 only because before then the embryo is incomplete. Subsequently to 

 this it cannot be delayed, because at the critical period, unless a new 

 form of nutrition be in existence, the embryo runs a danger of starv- 

 ation, if it remain in utero. The condition necessary for further 

 uterine life is, that prior to the critical period the formation of an 

 allantoic placenta should have been initiated, so that this organ may 

 begin to provide for the nutrition of the foetus, as soon as other 

 sources of nutrition, i. e. the trophoblast, begin to fail. As I 

 have previously pointed out, the trophoblast must, and does, begin 

 its degeneration, when the critical period is reached, and in all 

 the cases yet examined by others or by myself at this period the 

 allantoic placenta, if developed, begins its functions. It is not neces- 

 sary, that the allantoic placenta should be fully developed at the 

 critical period, but unquestionably in those cases I have studied there 

 is a slight placenta at the critical period, and in sheep and pig, as 

 will be demonstrated elsewhere , the capillaries of this begin to be 

 formed only slightly before the critical period. 



A year ago it was stated, that "an allantoic placenta is never, 

 and can never be, developed much, if at all, before the critical period 

 is reached" ^). 



Here two alternatives were suggested, that the placenta might 

 to be formed at the critical period, or that this might happen slightly 

 before the period. The latter is what is actually realised, and now it 

 appears to me even a priori the more likely. An allantoic placenta 

 is after all an organ of the embryo 2), and, as such, its foundation •'') 

 must be present by the time the critical period is reached, and, if 



1) Beakd, J., Certain Problems of Vertebrate Embryology, 1896, p. 46. 



2) To describe it as "an organ of the chorion" is no explanation. 

 Its initiation means the commencing degeneration of the latter. 



3) The essential part of the placenta is, practically, not the allantois 

 itself but the allantoic capillaries. 



