52 J. P. HTLL. 



tion products formed either directly from the wall-cells or 

 from these internal cells. 



2. Differentiation of the Embryonal Ectoderm and 

 the Entoderm. 



After the preliminary growth in size of the blastocyst is 

 coaipleted, the next most important step in the progi-essive 

 development of the latter is that just dealt with, involving 

 the appearance of the sutural line, with resulting re-establish- 

 ment of polar differentiation in the blastocyst wall. Following 

 on that, we have the extremely important period during 

 which the embr3'onal ectoderm and the entoderm become 

 definitely established. 



For the investigation of the earlier phases of this critical 

 period I have had at my disposal a large number of 

 unilaminar blastocysts derived from three females, dis- 

 tinguished in my notebooks as /3, 25 . vii . '01, with fifteen 

 vesicles of a maximum diameter of 4*5 mm. ; 8 . vii . '99, with 

 tAvelve vesicles, 4*5 ,mm. in diameter ; and 6 . vii . '04, with 

 twenty-two vesicles, 4*5 and 5 mm. in diameter. These three 

 lots of vesicles may for descriptive purposes be designated 

 as '01, '99, and '04 respectively. 



The '01 vesicles are distinctly less advanced than the 

 other two. The sutural line is now, at all events, definitely 

 continuous, and can readily be made out in the intact vesicle 

 with the aid of a low-power lens (PI. 4, fig. 38, j.l.), but. 

 the differences between the cellular constituents of the two 

 hemispheres which it separates are much less obvious than 

 they are in the '99 and '04 vesicles. Here, again, one 

 hemisphere forming half or perhaps rather more of the entire 

 vesicle is distinguished from the other by the greater uni- 

 formity and the slightly deeper staining character of its 

 constituent cells (figs. 43 and 44, tr. ect.). This hemisphere, 

 subsequent stnges show, is the lower or non-formative 

 hemisphere. It is characterised especially by the striking 

 uniformity in the size of its cells. Over the opposite hemi- 

 sphere, the upper or formative one (figs. 43 and 44, f.a.), the 



