88 J. BEARD, 



the foundations or "Anlagen" of all the organs; it is that state, when 

 epigenesis is ended, and evolution or unfolding is beginning ; it is that 

 point, where the individuality of the organism is first attained, when 

 it has acquired a something setting it down as the embryo of some 

 particular form, and — the wording is important — when it is first 

 beginning to resemble its progenitors. It then bears no ab- 

 solute likeness to them, but it is just beginning to look like 

 them 1). 



As already stated, a comparison of the table relating to the 

 opossum in my former memoir with that regarding Trichosurus in the 

 present contribution will convince the reader, that at birth these two 

 forms are practically in all essentials in corresponding phases of 

 development. This fact may, indeed , be accepted as having already 

 been established by the researches of Selenka for the opossum and 

 Hypsiprymnus, i. e. for an American and an Australian form. Certainly, 

 and I have formerly quoted the same passage , Selenka states that 

 "in der Ausbildung des Körpers und der Organe" the latter "etwas 

 zurücksteht" as compared with the opossum. The state of a few 

 organs at birth viz. lungs, mesonephros, and permanent kidney, is 

 recorded,>butJ^neither these, nor the newly-born form itself, present 

 any appreciable divergences, when compared with Trichosurus, and, 

 regarding the remaining organs, the reader is referred to the de- 

 scription of the newly-born opossum. Under these circumstances there 

 appears to be no escape from the conclusion, that in Hypsiprymnus 

 also the birth takes place at the equivalent of the critical period of 

 other forms. So far as the facts are available regarding Hypsiprymnus, 

 they all tend to prove this, and it may be asserted, that not one of 

 them is open to any other interpretation. The significance of this 



1) In certain forms it is a matter of great ease to pick out from 

 a fair series of progressive embryos the one , which must be in the 

 critical period. Of external characters the following, if they can be 

 got, are good land-marks to go by: — the mammary line has just 

 entirely disappeared , and has left a certain number of well defined 

 milk-points, the lachrymal groove has quite disappeared, hair-follicles are 

 visible on some parts of the face , the eyes are ovoid and no longer 

 rounded (Keibel), the fore-arm is flexed and the digits of the hand 

 are easily made out. Sometimes all these tests cannot be obtained, as 

 in the sheep, where a mammary line is never developed, and practically 

 hardly one of them can be applied to marsupial embryos. 



