224 TORREY 



post-trochal region, and becomes differentiated for the most part 

 into gut muscles (Text-Figs. 6, C, and 9, D). The teloblast- 

 hke cells, themselves, finally lie on each side of the oesophagus 

 and ^\v^ rise to a part of the ventro-cesophageal musculature. 

 I would call attention to the fact that Hatschek has described 

 cells similar in position, and probably also in fate, in Eiipomattis. 



The other ectomesoblast cell, 3^2. 2. 2» divides equally as it 

 sinks in (PL 11, Figs. 22 to 24), and after two or more unequal 

 divisions, the cells migrate toward the mid-ventral line (Text- 

 Fig. 9, A). The progeny of the cell, which after the first equal 

 division lay nearest the b quadrant, makes good the failure of 

 this region to produce an ectomesoblast cell (as stated on p. 201 

 the corresponding cell in this quadrant produces a large cesoph- 

 agoblast). (See Text-Fig. 9, C, cmui). The migration of this 

 cell in Podarke to a position symmetrical with the bilaterality of 

 the embryo is even more remarkable because it does not divide 

 until it lies in the future mid-ventral line and there equally and 

 bilaterally. In Uiiio, according to Lillie, the entire mesen- 

 chyme arises asymmetrically from 2(^o i+, but ''apparently by 

 active migration becomes symmetrical in later stages." What 

 causes this migration it is impossible to say, but it certainly 

 well illustrates the idea expressed by Lillie ('01) in the follow- 

 ing sentence : "That the entire organism at every stage of its 

 development exercises a formative influence on all its parts ap- 

 pears to me an absolutely necessary hypothesis." By the 

 shifting of the blastopore and the formation of the oesophagus 

 the progeny of these cells becomes carried up into the praetro- 

 chal region and there gives rise in large part to the muscles 

 running from the body wall to the oesophagus (Text-Figs. 6, 

 A and 7, E). 



The ectomesoblast cells from the first quartet sink into the 

 primary body-cavity somewhat later than those from the third. 

 The late period at which these cells are differentiated makes it 

 impossible to give their exact cell-lineage, but the general 

 regions, from which they arise, are as follows : Two cells sink 

 in on each side from the a and c intergirdle regions and just 

 above the prototroch (Text-Figs. 8, ii, and 9, B, le). They 



