576 J. T. PATTERSON 



is the homologue of the inner cell-mass of the eutherian blastocyst ; 

 and that the non-formative region is the homologue of the hypo- 

 blast of the eutheria and of the extra-embryonal ectoderm of the 

 sauropsida and monotremata. 



After the blastocyst has reached a diameter of from 4 to 5 mm., 

 two distinct varieties of cells can be recognized in the unilaminar 

 wall of the formative region. Hill regards this as the crucial stage 

 in the formation of the primary germ layers, as it marks the tran- 

 sition from the unilaminar to the bilaminar condition. We may 

 quote from Hill's summary such paragraphs as give a resume of 

 his account of the development of the entoderm. 



The formative region, unlike the non-formative, is constituted by 

 cells of two varieties, viz. : (i) a more numerous series of larger, lighter- 

 staining cells destined to form the embryonal ectoderm, and (ii) a less 

 numerous series of smaller, more granular, and more deeply staining 

 cells, destined to give origin to the entoderm and hence distinguishable 

 as the entodermal mother-cells. 



The entodermal mother-cells, either without or subsequent to division, 

 bodily migrate inwards from amongst the larger cells of the unilaminar 

 wall and so come to lie in contact with the inner surface of the latter. 

 They thus give origin to the primitive entodermal cells from which the 

 definitive entoderm arises. The larger passive cells, which alone form 

 the unilaminar wall after the inward migration of the entodermal cells is 

 completed, constitute the embryonal ectoderm. 



The entodermal cells as well before as after their migration from the 

 unilaminar wall are capable of exhibiting amoeboid activity and of 

 emitting pseudopodial processes, by the anastomosing of which there is 

 eventually formed a cellular entodermal reticulum underlying, and at 

 first coextensive with, the embryonal ectoderm. 



The entoderm is first laid down below the formative or embryonal 

 region of the blastocyst; thence it extends gradually by its own growth 

 round the inner surface of the unilaminar non-formative region so as to 

 form eventually a complete entodermal lining of the blastocyst cavity. 

 In this way the blastocyst wall becomes bilaminar throughout. 



Thus it will be seen that, contrary to the generally accepted view 

 of mammalian embryologists, the entoderm of Dasyurus does not 

 arise by delamination, but through an inward migration of differ- 

 entiated entodermal mother-cells from among the ectoderm cells 

 of the embryonic region. The independent discovery of a similar 

 method of entodermal formation in the armadillo is of more than 

 ordinary interest, and calls for a detailed account of the process in 



