ON THE DEVELOPMENT OF NEBALTA. 397 



the caudal tliickening. They show, in the region occupied 

 by the groove in Stage B, a band of large rounded cells, the 

 endoderm, resulting from the invagination. On either side 

 of this band in its anterior region the body wall consists of 

 two layers of cells, ectoderm and mesoderm, since there are 

 here the posterior convergent ends of the thickened lateral 

 strands (fig. 11). 



Unfortunately the sections of the series here drawn are a 



Mas. 



DiAGiiAM 1. — Transverse section tliroufjli the caudal tliickenin«: 

 in Stage B', showing the remains of the groove "roofed in" 

 by tlie closure of the blastopore. Ec. Ectoderm. En. Eiidoderm. 

 Mes. Mesoderm. 



little oblique, or they would show more distinctly than they 

 do the remains of the groove which is now roofed in, as it 

 were, by the closing of the blastopore, as shown in the 

 above diagram. 



It can be seen from figs. 12 and 13 that the endoderm cells 

 have already begun to proliferate ; and from figs. 13 and 14 

 it would appear that their numbers were also increased by 

 cells being pushed in from the ectoderm. Further back it 

 becomes rather difficult to distinguish between the endoderm 

 and the mesoderm ; but there is a difference in the shape of 

 the cells, those of the mesoderm being oval or almost almond- 

 shaped, whilst the endoderm cells are much more nearly 

 spherical. 



All the protoplasm in this stage is very granular, and the 



