50 CARL GOTTFRIED HARTMAN 



where they deviate from this shape in the }3reparations they 

 have suffered collapse to that extent. In the older of Selenka's 

 two gastrulae the enormous 'Perivitellinraum' between the blasto- 

 cyst and its albumen layer is to be regarded with suspicion. I 

 have never seen a blastocyst, the cells of which were not in close 

 contact with the albumen layer. Judging from the quantity of 

 albumen remaining in Selenka's figure 1, Tafel 18 (although it 

 is a rough criterion at best) I regard his older gastrula there 

 figured to be a shrunken specimen of a stage just preceding my 

 own No. 40 (4), shown in outline in text figure 6 D and in detail 

 in figure 32; and this in turn is a stage just preceding Selenka's 

 almost complete bilaminar blastocyst which he shows in figure 

 3, Tafel 18. 



It is evident then, that Selenka did not, on account of a lack 

 of the requisite stages, hit upon the true method of entoderm 

 formation in the opossum. 



b. Entoderm formation in Dasyurus 



In Hill's account of the history of the blastocyst of Dasyurus 

 he gives us a new method of entoderm formation in mammals, 

 and his interpretation of the facts as he observed them in Dasy- 

 urus finds confirmation in similar conditions independently dis- 

 covered by Patterson CIO) in the embryo of the Tatusia. 



In Dasyurus there appears in the still unilaminar wall of the 

 blastocyst, when this has attained a diameter of nearly 4.0 mm., 

 a number of modified ectodermal cells, the 'entoderm mother 

 cells.' These produce the entodermal cells by a process of pro- 

 liferation and migration on to the inner surface of the blastocyst 

 wall. They appear at the formative pole of the blastocyst and 

 at an early stage are united into a fenestrated tissue by means 

 of pseudopodial prolongations. Eventually they form the entire 

 inner lining of the blastocyst. 



Hill's conclusions are borne out by young blastocysts of other 

 Australian forms. In all cases the germ layer formation differs 

 essentially from that obtaining in Eutheria in the absence of the 

 morula stage. That the same facts are substantially true in 

 the development of the opossum I now propose to show. 



