FORxMATION OF GERMINAL LAYERS IN CHELONIA. 31 



Fig. 20 passes slightly in front of the ventral opening of 

 the blastoporic passage. In the median line the chorda-ento- 

 blast (enc.) forms directly the roof of the digestive cavity^ 

 without the intervention of the darm-eutoblast {end.) which 

 stops at a short distance from the axial line. On the left side 

 of the section, more clearly than on the right, the darm-ento- 

 blast is seen to make a fold at its innermost point where it 

 abuts against the chorda-entoblast and then to turn outside 

 asrain to be lost in the mesoblast. The mesoblast is therefore 

 partly continuous with the chorda-entoblast and partly with 

 the darm-entoblast. In other words, it starts from the point 

 where the chorda- and the darm-entoblast meet each other. The 

 mesoblast cells in this region show a peculiar arrangement. 

 Those cells next the ectoblast are columnar and look like the 

 continuation of the chorda-entoblast. The cells placed ven- 

 trally to these are polygonal and without any definite arrange- 

 ment. Laterally cells are being added to the mesoblast in the 

 whole region of the germinal wall, but especially at a ; proli- 

 feration 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 cells, and never extends to the inner part which 

 has only a single layer of cells, and constitutes the well 

 differentiated darm-entoblast. 



We pass over for the present figs. 2] and 22, and come to 

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

 region of the head-fold as shown by a notch (A./.) 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 the 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 tiie lateral mass 

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



