FOKMATION OF GERMINAL LAYERS IN CHELONIA. 227 



definite arrangement. Laterally cells are being added to the mcso- 

 blast in tlie whole region of the germinal wall, but especially at a ; 

 proliferation seems to take place in the posterior region even from the 

 outer part of the darm-entoblast, as in this section. This is, however, 

 confined to the part which still consists of two or three layers of cella, 

 and never extends to the inner part which has only a single layer of 

 cells, and constitutes the well difi'erentiated darm-entoblast. 



We pass over for the present figs. 21 and 22, and come to the 

 most anterior section represented (fig. 23). It is in the region of the 

 head-fold as shoM'n by a notch (/;./.) on one side in the ectoblast. 

 The darm-entoblast, which is laterally quite thick and consists of 

 columnar cells, is internally very thin and becomes continuous with 

 tlie chorda-entoblast near the median line. From the point of junction 

 as well as from the sides of the chorda-entoblast mesoblastic cells are 

 budding off on each side. There is in this section a small mass of 

 mesoblast cells outside of the head-fold which is distinct from the 

 main mass. This isolation has been brought about by the ectoblast 

 folding downward as the head-fold; more posteriorly the lateral mass 

 fuses with the main mass. In this section the germinal wall is 

 absent, and thus no additions are made laterally to the mesoblast from 

 the entoblast. 



Returning to the middle region of the body, figs. 21 and 22 

 serve to show the first steps in the formation of the notochord. The 

 chorda-entoblast which in fig. 20 passed laterally without any inter- 

 ruption into the mosoblast, is in fig. 21 marked oft" from the meso- 

 blast, at least in the upper part. The cells at the border between the 

 two are turned away from one another ; thus the cells of the chorda 

 are directed inwards and downwards, while the contiguous cells of the 

 mesoblast are directed outwards and downwards. The mesoblast is 

 still united with the darm-entoblast. As yet, the chorda is only a 



