THE ARCHENTERIC INVAGINATION. 209 
which 1s effected in the vesicular blastoderm manifests 
itself on that face of it which is turned towards the pedicle 
of the egg. Here the layer of cells becomes thickened 
throughout an oval area about 1-25th 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, bp) appears 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 which 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, bp), diminishes in size. 
Thus the body of the embryo crayfish, 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 
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