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 résumé 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 
