84 • J. BEARD, 



to the list published in a former memoir 0, the characters of the two 

 embryos have been drawn up in tabular form, and the resulting table 

 has been appended to the present writing. 



It will be noticed both from the table and from the description, 

 that the newly-born Trichosurus possesses only organs and foundations 

 of organs present before birth, i. e. in the uterine specimen. This is 

 a result of some importance; for, when taken along with the fact, that 

 in the new-born form all the foundations of organs are in existence 

 and none wanting , it shows, that the animal is not born , until the 

 foundations of all the organs are laid down, i. e. until the embryo is 

 complete. 



Furthermore, it is clear, that, whilst in some respects the 

 newly-born specimen presents an advance on the uterine embryo, 

 the differences, though important, are but slight, and only appreciable 

 ones in the cases of a few of the organs. 



This proves, that the gap between the two forms is a very narrow 

 one, and this is also established by other considerations. The two 

 specimens are very similar in size, the apparent difference being 

 practically entirely due to the raising of the head in the pouch-foetus. 

 The birth could only have been quite recent, when the foetus was 

 preserved ; for the umbilical cord was still largely existent, and its 

 component cells had not shrivelled up. Moreover, although the milk- 

 nutrition had begun , and although the gut contained milk , no 

 complete digestion of this milk had taken place. 



All these things together furnish a body of evidence tending to 

 prove the interval between the two embryos to be a very small one, 

 the one embryo standing only a little beyond the birth-point, the other 

 only slightly before it. 



This is a result of extreme importance; for, as will be presently 

 demonstrated, it establishes the coincidence of birth- and critical periods 

 in Trichosurus vulpecula. 



The uterine embryo falls very slightly short of fulfilling the con- 

 ditions of the critical phase, now established in a number of other 

 forms. Though the foundations of all its organs are present, certain 

 requisites of the critical period are still lacking. Thus, the head is 

 not raised, the fore-arm is not sufficiently flexed, the lachrymal duct 

 does not open into the nasal chamber, the formation of the posterior 

 fissure is not initiated, and the sex is not established. 



1) On certain Problems of Vertebrate Embryology, Jena 1896, 

 p. 72—77. 



