544 William A. Hilton. 



the other mass of cells and probably occupies a position in part 

 corresponding to the future position of the archenteron. The arch- 

 enteron in some early stages seems to become quite a cavity ; the living 

 egg gives no indication of the cavity, but it was easy to injure its 

 thin roof on examining the egg, so it was often accidentally demon- 

 strated. In preserved eggs this thin layer of cells sinks down toward 

 the lower side of the cavity as shown in Fig. 59. In stages of this 

 kind the blastopore remains as a small often triangular opening at 

 what may be called the caudal end of the egg and very faint indica- 

 tions of a broad medullary plate make their appearance, Fig. 30. 

 A longitudinal section of such a stage cut through the blastoporic 

 opening shows the thin roof of two general layers, somewhat collapsed, 

 but still with a considerable archenteron. The cells of the ventral 

 lip of the blastopore are shown as slightly different from the general 

 yolk cells and to some degree forming the floor of the archenteron. 



In these last stages there has been* no reference to a segmentation 

 cavity because none was clearly recognized. Spaces sometimes occur 

 between the cells of the yolk mass, possibly in some cases artifacts, 

 but in some at least they seem to be secondary segmentation cavities 

 developed between the blastomeres. 



From the material at hand it was impossible to recognize with 

 certainty the cavities in the egg at the time of gastrulation which 

 Schultze recognizes in the frog. In Desmognathus there is at first 

 the segmentation cavity or blastocele, but it seems to entirely dis- 

 appear before gastrulation. Later a secondary segmentation cavity 

 can be recognized as shown in Figs. 58 and 59, and the cavity of the 

 archenteron is apparent. The secondary cavities which make their 

 appearance between the yolk cells in later stages may correspond to 

 the "Erganzungshohle" of O. Schultze. 



Very soon after the stage described as yolk plug, or about forty 

 hours after laying, there comes to be a very slight indication of the 

 medullary plate from the exterior, in the large area of the egg which 

 floats up because it is lighter. It is whiter than other parts of the 

 egg. In the central part of this area there soon comes to be a very 

 shallow gi'oove and on each side of this a gentle slope of the very 

 broad early medullary plate. This slight groove leads down to the 



