56 J. p. HILL. 



the largest of the fonnative ; their nuclei lie nearer each other, 

 Avith the result that in surface examination of the blastocyst 

 the non-formative region appears rather denser than the 

 formative. In in to to pi'eparations of the wall the former 

 usually stains darker than the latter (fig. 45), but this is not 

 always the case ; in fig. 46, from an ^04 vesicle, there is 

 practically no difference in this respect between the two 

 regions ; in yet others of my preparations of ^99 vesicles the 

 formative region has stained more deeply than the non- 

 formative. 



The formative hemisphere in the earlier blastocysts of this 

 particular developmental stage was described (ante, p. 51) as 

 differing from the non-formative in that its constituent cells 

 were much less uniform in character than those of the latter. 

 This same feature, but in much enhanced degree, characterises 

 the formative region of the vesicles under consideration, for it 

 can now be definitely stated that the latter region is consti- 

 tuted by cells of two distinct varieties, viz. (1) more lightly 

 staining cells which form the chief constituent of the forma- 

 tive region, its basis so to speak, and which are on the 

 average larger than those of the other variety, and (2), a less 

 numerous series of cells, distinctly smaller than the largest 

 cells of the former variety, and with denser, more granular and 

 more deeply staining cytoplasm, and frequently met with in 

 mitotic division (cf . PI. 6, fig. 65). The two varieties of cells are 

 intermingled promiscuously, the smaller cells occurring singly 

 and in groups but in a quite irregular fashion, so that here 

 and there we meet with patches of the wall composed exclu- 

 sively of the larger cells. 



The evidence presently to be adduced shows that the larger 

 cells furnish the embryonal ectoderm, and that the smaller 

 cells give origin to tlie primitive entodermal cells from which 

 the definitive entoderm arises. The smaller cells may there- 

 fore be regarded as entodermal mother-cells. Whether these 

 latter cells are progressively formed from the larger cells 

 simply by division, or whether the two varieties become 

 definitely differentiated from each other at a particular stage in 



