584 J. T. PATTERSON 



Concerning the idea of separating the processes of gastrulation 

 and notogenesis I believe that in the main we must agree with 

 Hubrecht, although the close genetic continuity of these two proc- 

 esses must never be lost sight of, even in mammals, else the ap- 

 pearance of an evanescent blastopore in such forms as the hedge- 

 hog, Tarsius, rabbit, mole, shrew, and opossum would have little 

 significance. But the unexpected discovery of migrating cells to 

 form the entoderm in the blastocyst of the armadillo must deter 

 us from speaking of the process of entoderm formation as one of 

 delamination, for this term as used in mammalian embryology im- 

 plies that the entoderm is differentiated from those cells of the 

 inner cell-mass which happen to border on the cavity of the vesicle. 

 So to regard the origin of the entoderm would be equivalent to ac- 

 cepting Driesch's ('93) aphorism; for it would amount to saying 

 that the prospective value of one of these bordering cells "is a 

 function of its position. " Delamination is not therefore the cor- 

 rect term to employ in describing the -mode of entoderm forma- 

 tion in the armadillo, for while it is true that the entoderm as a 

 distinct layer is split off from the embryonic ectoderm, as we shall 

 see later, yet prior to this so-called delamination there is an un- 

 mistakable migration of primary entodermal cells to the lower 

 surface of the inner cell-mass, and this it seems to me is the funda- 

 mental step in the whole process of entoderm formation. 



The final steps in the differentiation of the entoderm in the arma- 

 dillo may be considered here, as we shall not have occasion again 

 to refer to them. Following the conditions such as we have seen 

 in figure 41 the entodermal cells spread out until they have formed 

 a continuous sheet of cells beneath and coextensive with the em- 

 bryonic ectoderm (fig. 47). During this change the entodermal 

 cells become much flattened against the ectoderm. There is no 

 evidence here, such as we have observed in the pseudopodia of 

 earlier stages, to indicate that the entodermal cells at the margin 

 of the embryonic area are pushing out further beneath the tropho- 

 blast. Even in the specimen shown in figure 42 the pseudopodia 

 have practically all disappeared, except in the case of the upper 

 marginal cells that border on the area of ectoderm still not covered 

 by entoderm (fig. 12). 



