206 HUBERT DANA GOODALE 



except near the notochord, n, where it is not yet quite separate. 

 Its rounded cells are arranged, roughly speaking, into two layers. 

 Those of the inner layer are often smaller and somewhat elongated. 

 By joining end-to-end, they give indications of forming a definite 

 layer, the forerunner of the splanchnopleure. The archenteron, 

 a, has become very small. The number of cells bounding it is 

 very much less than before. In the mid-dorsal line the row of 

 notochord cells, n, begins to arch up. 



It is not my purpose to extend the account of the development 

 of the gastral mesoderm, since its origin, by a process of delami- 

 nation among the cells of the archenteric roof has been clearly 

 shown, although in later stages, the cells of the mesoderm come 

 to look very much like ectoderm. The coelom does not appear 

 until much later. 



The decrease in size of the archenteron must be commented 

 upon. Part of this decrease appears to be taken up by the decrease 

 in size of the egg which takes place at this time. The remainder 

 may be taken up by an imbibition of the archenteric fluid by the 

 yolk cells, so that they become larger. This appears to be the 

 case. But aside from the mere decrease in size of the archenteron, 

 a much more important question may be asked. WTiat becomes 

 of the yolk cells bounding the archenteron in the earlier stages? 

 Compare fig. 38 or 40 with 41 . Do the cells simply pull away from 

 the edge and become lost in the general mass of yolk cells or do 

 they follow some other course? While it is impossible to decide 

 between these two views, there is some evidence, i.e., the varjdng 

 shape of the cells which supports the former view. But without 

 denjang this view or asserting that the suggestion about to be 

 made is correct, I wish to point out one or two things: First, the 

 cells bounding the dorsal and lateral walls of the archenteron in 

 fig. 41, are as large if not larger than those in the earlier stages. 

 Second, the mesoderm has undergone a considerable extension 

 ventrally without any decrease in thickness and very little de- 

 crease in the size of the cells. These facts induce me to believe 

 that some of the floor cells of the archenteron (fig. 41, for example) 

 later come to lie in the lateral walls. The cells of the lateral 

 walls, in turn, go to furnish the material for the extension of the 

 mesoderm. There is no conclusive evidence that such is the case. 



