The Development of the Alhgator 235 



mentioned, where, medially, the ectoderm, meso- 

 derm, and entoderm form a continuous mass of 

 cells. Laterally the mesoderm (mes) is a distinct 

 layer of cells of a fairly characteristic mesodermal 

 type. The notochord is not yet discernible, 

 though a slight condensation of cells in the middle 

 line may indicate its position. 



Figure 2/ is one of the four sections that were cut 

 through the blastopore (blp), which is a hole of 

 considerable size that opens, as the figure shows, 

 entirely through the blastoderm. Along the walls 

 of the blastopore the ectoderm and entoderm are, 

 of course, continuous with each other and form 

 a sharply defined boundary to the opening. As 

 we pass laterally from the blastopore the cells 

 become less compact, and are continued on each 

 side as the mesodermal layer {mes) . In this series 

 the sections posterior to the blastopore were 

 somewhat torn, and so were not drawn; but they 

 probably did not differ materially from those 

 of the corresponding region of the immediately 

 following stages, which are shown in Figures 3m 

 and 6i and will be described in their proper order. 



STAGE II 



Figures 3-30 (Plate VII., VIII., IX,) 



The next stage to be described is shown in sur- 

 face views in Figures 3 and 3a. Of this stage Clarke 

 says: 



