THE ARCH ENTERIC INVAGINATION. 209 



which is effected in the vesicuhir blastoderm manifests 

 itself on that face of it which is turned towards the pedicle 

 of the egg. Here the laj'er of cells becomes thickened 

 throughout an oval area about l-25tli of an inch in 

 diameter. Hence, when the egg is viewed by reflected 

 light, a whitish patch of corresponding form and size 

 appears in this region. This may be termed the ger- 

 minal disk. Its long axis corresponds with that of the 

 future crayfish. 



Next, a depression (fig. 58, A, hj)) a]3pears in the hinder 

 third of the germinal disk, in consequence of this part 

 of the blastoderm growing inwards, and thus giving rise 

 to a small wide-mouthed pouch, which projects into the 

 food-yelk with w'hicli the cavity of the blastoderm is 

 filled (fig. 57, B, mg). As this infolding, or invagination 

 of the blastoderm, goes on, the pouch thus produced 

 increases, while its external opening, termed the blasto- 

 •pore (fig. 57, B, and 58, A — E, hp), dimmishes in size. 

 Thus the body of the embiyo cra3"fish, from being a 

 simple bag becomes a double bag, such as might be 

 produced by pushing in the wall of an incompletely 

 distended india-rubber ball with the finger. And, in 

 this case, if the interior of the bag contained porridge, 

 the latter would very fairly represent the food-yelk. 



By this invagination a most important step has 

 been taken in the development of the crayfish. For, 

 though the pouch is nothing but an ingrowth of part of 

 the blastoderm, the cells of which its wall is composed 



