128 



T. H. MORGAN. 



the dorsal lip of the blastopore lies. At the same time the 

 lateral wall of the blastocoel has been reduced in thickness, and 

 there can be little doubt that the reduction must be directly due 

 to the movement downwards of its cells. On the opposite side 

 of the section where the ventral lip will form later the down- 

 growth of small cells is less marked, and the lateral wall of the 

 blastocuel is correspondingly thicker. In other words, the ring 

 of embryo-forming material lies at this time obliquely on the egg 

 one side having pushed down further than the other. On the 

 right and left sides of the embryo, as seen when cross-sections 

 are made, the ring shows all intermediate stages between the 



o o 



condition at the dorsal and at the ventral lip. 



12 



13 '-! 15 



FIGS. 11-15. Roof of segmentation cavity of Rana palustris. 



The later history of the gastrulation has been sufficiently de- 

 scribed in former papers. In general, the process consists of 

 the steady migration of the embryonic ring over the lower hemi- 

 sphere. As this takes place the yoke is thrown high up into the 

 segmentation cavity, and the roof of the latter becomes reduced 

 to a thin layer of cells. 



The steady decrease in thickness of the roof of the segmenta- 

 tion cavity during the cleavage, pregastrula and gastrulation 

 stages is a constant feature of the development. The decrease 

 in thickness is well shown in the series of figures, Figs. 11-13, 



