' SEGMENTATION CAVITY ' OF RANA 37 



of the endodcrni intervening between the primitive enteron and 

 the secondary enteron. The intervening cells are thereby 

 reduced to a single layer. I have also a series of transverse 

 sections of this stage. This process creates an extension of 

 the primitive enteron and provides for the expansion of the 

 secondary enteron. The expansion takes place by the thin 

 floor being depressed into the primitive enteron, and during the 

 process the intervening cells are absorbed. 



Figs. 9 to 14 (PI. 1) are from a series of transverse sections 

 of the stage (Text-fig. 6) after the fusion of the two cavities 

 has taken place, and are photographs of sections from slides 3, 

 4, 5, 6, 8, 9. It will be seen that they bear no evidence of the 

 loss of the primitive enteron ; the cavity is practically as large 

 as ever. Incidentally they show that the notochord is con- 

 tinuous with the rest of the mesoderm, and that it is formed 

 by splitting off from the lateral wings of mesoderm. These 

 latter completely encircle the wide anterior region of the 

 enteron and already begin to exhibit the splanchnocoel. 



But the process of fusion is better illustrated by the trans- 

 verse sections of the younger stage, figs. 15 to 18 (PI. 1), 

 from slides 11, 12, 13, 15 — a stage intervening between Text- 

 figs. 5 and 6. In the first of the sections photographed, fig. 15 

 (PI. 1), the secondary enteron is seen above, not far from the 

 dorsal lip, and the primitive enteron below. The succeeding 

 figures demonstrate that the two cavities are converted by 

 fusion into one cavity. It will be observed that even at this 

 stage the mesoderm has made rapid progress in enveloping the 

 endoderm, and not by a process of delamination. 



I have preparations besides which show in sagittal section 

 the fusion depicted especially by Schulze and Hertwig (Text- 

 fig. 5). These, with the transverse sections submitted, prove 

 that the thin wall of cells which intervenes between the two 

 cavities is depressed and absorbed during the process. Some 

 degree of variation is possible as to the completion of a process 

 which I am led to regard as the normal one. If it is not the 

 normal process then a demonstration to the contrary is called for. 



The morphological aspects of the results I do not intend to 

 discuss. But I may be allowed to point out that my observations 



