74 .[. r. HLLL. 



wall aliords an iiitelligihle explanation of tlie niecliaiiisiii of 

 their inward migration. 



(7) The entoderm is first laid down below the formative or 

 embryonal region of the blastocyst; thence it extends gradu- 

 ally by its own growth round the inner surface of the uni- 

 laminar non-formative region so as to form eventually a 

 complete entodermal lining to the blastocyst cavity. In this 

 way the blastocyst wall becomes bilaminar throughout. 



(8) The bilaminar blastocyst consists of two regions, respec- 

 tively embryonal and extra-embryonal. The embryonal 

 region (embryonal area) is constituted by an outer layer of 

 embryonal ectoderm and the underlying portion of the ento- 

 derm, and the extra-embryonal, ot the extra-embryonal or 

 trophoblastic ectoderm (tropho-ectoderm), which is separated 

 from the embryonal by a well-marked junctional line, together 

 with the underlying portion of the entoderm, which is per- 

 fectly continuous with tiiat below the embryonal ectoderm. 



(9) The formative or embryonal region of the blastocyst 

 in Dasyurus is from the first freely exposed, and at no time 

 during the developmental period dealt with in this paper 

 does there exist any cellular layer externally to it, i.e. a 

 covering layer of trophoblast (Deckschicht, liauber's hiyer) 

 is absent and there is no entypy of the primary germ-layers 

 (cf. p. 111). 



CiiAPTEii Y. — Some Eai;j,y Stages of 1'ekameles and 

 Mackopus. 



The early material of I'erameles and Macropus at my 

 disposal comprises only a small nuniber of stages, but is of 

 special importance, since it enables me to demonstrate tliat 

 so far as these particular stages are concerned, the early 

 developmental phenomena in these forms are essentially the 

 same as in Dasyurus, and thus affords ground for the belief 

 that there is one common type of early deveU)))ment through- 

 out the series of the j\Jarsupialia. Moreover, it is of interest 

 since it reveals the existence of what mioht be termed 



