10 THE DEVELOPMENT OF 



but it seems probable that they are also entodermic, and 

 eventually give rise to the epithelium of the mesenteron. If 

 the above explanation of Nusbaum's account of the develop- 

 ment of Mysis be correct, his entoderm is nothing of the 

 sort and his vitellophags are apparently the only cells 

 which can supply the lining of the mid gut. Until he pub- 

 lishes his complete account one is left in doubt as to the 

 evidence he has that these cells degenerate and disappear. 



STOMODEUM AND PROCTODEUM. Both stomodeum and 

 proctodeum are well marked structures in stage B (fig. 

 11). At this time the proctodeum is a well developed ec- 

 todermal invagination (fig. 36) arising behind the abdom- 

 inal flexure and formed of cells more or less columnar in 

 appearance. Its inner extremity terminates blindly, abut- 

 ting directly against the yolk, having apparently pushed 

 aside the mesoderm which formerly (fig. 29) formed an 

 unbroken sheet in this region. In the adjacent portion of 

 the yolk are several entoderm cells placed closer to each 

 other than in other parts of the deutoplasm. Behind the 

 proctodeum are a few mesodermal cells, while in front 

 (morphologically below) this layer is much more exten- 

 sive, reaching forward as a connected sheet (see below 

 under mesoderm) around the abdominal flexure to beyond 

 the stomodeum. In these points my studies are but little 

 more than a confirmation of those of Reichenbach. 



The stomodeum at this stage is less deep and it is to 

 be noted that it has not yet broken through the mesoderm 

 nor does it do so until a much later stage. It is also to be 

 noted as is also the case in other forms, that it has not that 

 flexure so characteristic of it in its later stages. In the 

 section figured it has no well marked lumen but in others 

 (which do not show other features so well but which are 

 less contracted by the hardening reagents) it has a much 

 larger cavity in proportion than has the hind gut. At 



