THE EARLY DEVELOPMEXT OF THE MARSUriALTA. HI 



homologue of the inner cell-mass, gives origin not only to the 

 embryonal ectoderm, but to the entire entoderm, wliilst the 

 non-formative region, whose homology to the troplioblast of 

 Eutheria is admitted by Assheton, arises quite independently 

 of the entoderm and a long time before the latter makes its 

 appearance. There is, then, in Dasyurus no question of a 

 common origin of the entoderm and the non-formative or 

 trophoblastic region of the blastocyst wall. And exception 

 may be taken to Assbeton's views on quite other giounds 

 (e. g. the question of the homologies of the foetal membranes 

 in the series of the Amniota), as he himself is well aware, and 

 as Jenkinson ('00) has also emphasised. I feel, however, I can 

 leave further discussion of Assbeton's views until such time 

 as my observations on Dasyurus are shoWn to be erroneous or 

 inapplicable to other Marsupials. 



•3. The Entypic Condition of the P]utherian 

 Blastocyst. 



If, now, on the basis of the homologies I have ventured to 

 advocate in the preceding pages, we proceed to compare the 

 Metatherian wMth the Eutherian blastocyst, we have to note 

 that, wherens in the latter the extra-embryonal or tropho- 

 blastic ectoderm alone forms the blastocyst wall in early 

 stages and completely encloses the embryonal knot, in the 

 former, the homologous parts, viz. the non-formative or extra- 

 embryonal and the formative or embryonal i-egions, both 

 enter into the constitution of the unilaminar blastocyst 

 wall, there being no such enclosure of the one by the 

 other as occurs in the Eutherian blastocyst (Text-fig. 2, p. 98). 

 It is characteristic of the Marsupial as of the Monotreme that 

 the embiyonal region is from the first superficial and freely 

 exposed. It is spread out as a cellular layer and simply 

 forms part of the blastocyst wall or blastoderm. It is equally 

 characteristic of the Eutherian that the homologous part, the 

 embryonal knot, has at first the form of a compact mass, 

 which is completely enclosed by the trophoblastic ectoderm. 



