1 36 MCMURRICH. [Vol. XI. 



contact with a posterior prolongation of the mesenteron, which 

 Reichenbach describes as being formed from the entoderm- 

 plate cells. The ventral wall of the mesenteron is formed of 

 similar cells, as is also a certain amount of the dorsal wall, and 

 it would require but a comparatively slight extension of the 

 yolkless cells to complete the mesenteron and shut off from it 

 completely the yolk-pyramids. 



Unfortunately, observations on the later stages of Astacus 

 are wanting to render such an interpretation of the formation 

 of the mesenteron a certainty, but I hold that it is one which 

 is more in harmony with the cases in which the fate of the 

 vitellophags has been fully traced up to the disappearance of 

 the yolk, than the view which is generally held. And, further- 

 more, it harmonizes with certain observations of Reichenbach 

 himself on the yolk-pyramids. It is easily recognizable that as 

 development proceeds in Astacus there is a marked diminution 

 of the number of secondary yolk-pyramids, and there is good 

 reason to suppose that this is associated with the formation of 

 the so-called secondary mesoderm cells ; indeed, Reichenbach 

 has traced the formation of these cells to the yolk-pyramids. 

 In part, then, the yolk-pyramids give rise to mesoderm elements, 

 and their entire conversion into such elements does not seem 

 to me at all improbable. 



If these views be correct, then we have in Astacus, and 

 probably in Eupagurus as well (Mayer, '77), an invagination 

 which includes both endodermal and mesodermal elements, and 

 there is a further formation of mesoderm by immigration im- 

 mediately in front of the invagination. This condition is 

 derived from one in which no early differentiation of endoderm, 

 mesoderm, and vitellophags can be recognized, and from this 

 same condition is to be derived the arrangement seen in Jacra, 

 where the ordinary mesoderm and endoderm form a common 

 undifferentiated mass while the vitellophags are sharply marked 

 off, as well as the arrangement described for Lucifer by Brooks 

 ('83), in which there is an invagination of endoderm and the 

 differentiation of two cells which migrate into the blastocoel 

 before invagination, and apparently represent both the meso- 

 derm and vitellophags. 



