110 THE DEVELOPMENT OF 



The more prominent of the internal features of the gas- 

 trulation may be seen in üg. 9, whicli represents an ob- 

 lique section through a stage interinediate between those 

 shown in surface views. The endoderm cells {h) are be- 

 ing forced almost vertically into the yolk, though with an 

 inclination tovvards the anterior end of the eo^o^. The nu- 

 clei are placed at the deeper ends of the cells, the proto- 

 plasm of vvhich Stretches upwards to the blastopore. In 

 the Upper ends of these endoderm cells the boundaries be- 

 tween the cells can be seen with some distinctness as is 

 shown in the figure, but deeper they entirely disappear. 

 This invaginated endoderm is a solid mass and contains no 

 lumen, or archenteron, and the blastopore itself is but a 

 depression in the general surface of the egg. The subse- 

 quent fate of these cells will be traced later ; but here we 

 may say that they soon separate from the parent layer and 

 sink into the yolk where they divide into two groups, 

 a few going to the region where the stomadseum is subse- 

 quently to form, while the greater portion do not move far 

 from the point of their differentiation and later unite with 

 their fellows and with theproctodaeum. These cells here, 

 as in other species,form the mesenteron, the cavity of which 

 exists, until after hatching, only in a potential condition, 

 being filled completely with the deutoplasm. Through all 

 of the larval stages these endodermal cells can be readily 

 recognized by their larger size and by the fact that their 

 nuclei stain less deeply than those of mesodermal or ecto- 

 dermal origin. 



Fig. 9 also shows some features in the origin of the 

 third germinal layer, the mesoderm. On either side of the 

 endodermal invagination may be seen some cells with large 

 nuclei and amoeboid outlines, which are plainly budding 

 from the cells at the mouth of the blastopore and sinking 

 into the yolk. Owing to the great difficulties encountered 

 in orienting the eggs at this early stage, I cannot say that 



