The Birth-period of Trichosurus vulpecula. 85 



In other points the embryo fulfils the requirements of the critical 

 period, and in those, in which it is wanting, the advance required to 

 bring it to the critical period could a priori be very quickly 

 effected. 



It may, therefore, be concluded, that the uterine embryo A is 

 just verging on the critical period. The new-born foetus B, on the 

 other hand, fulfils all the requirements of the critical period, but it 

 does rather more. As marking the critical period may be noted the 

 raising of the head, the complete flexure of the fore-arm, the opening 

 of the lachrymal duct into the nasal chamber, the establishment of 

 sex, and, most important of all , the commencing formation of the 

 posterior fissure of the cord. My work of the past year, which has 

 covered a large ground , and which for that reason alone cannot be 

 quickly published, has, among other results, revealed, that in those 

 cases, in which the beginning of the new mode of nutrition can be 

 definitely fixed, this point marks the end of the critical period. 



In the newly-born foetus of Trichosurus the mammary nutrition 

 has been established, but only recently, and certainly for not more 

 than 2 or 3 hours previous to the death of the foetus. Moreover, 

 the formation of the posterior fissure is not merely initiated, it has 

 advanced slightly beyond the starting point, and may be compared, as, 

 indeed, the total of the organisation of the foetus may also be, with 

 the condition in a Scyllium canicula embryo, in which yolk has just 

 entered the spiral valve, the homologue of the small intestine. 



Thus, it appears, that this foetus has just emerged, or is just 

 emerging, from the critical period ; and, as the unborn embryo is just 

 about to enter it, there being but slight differences between them, it 

 is thereby established that in Trichosurus vulpecula the critical phase 

 must begin immediately before birth takes place, and that it must 

 end soon after the animal is born, within an hour or two of birth, 

 with the initiation of the new nutrition, that afforded by the mammae. 

 The result may be stated shortly in these terms: the critical and 

 birth-periods coincide in Trichosurus vulpecula. 



This statement is absolutely correct, when the sum-total of the 

 organisation of the Trichosurus -^mhrjo at the birth -period is 

 taken into account, but it must be pointed out, that in one or two 

 respects the marsupial presents differences, when compared with a 

 critical rabbit, pig, sheep, or even with a dog-fish of the critical period. 



These divergences might become difficulties, if they were in 

 essential points, more particularly if they related to the nervous 



