VIII 



AKTHROPODA 



191 



look at the nearer allies of Astacus, we find that in Homarus (the 

 lobster) the egg is much larger than that of Astacus, owing to the 

 presence of a larger amount of yolk, and that the endodermic area 

 is relatively small. The iuvaginated cells form at first a nearly solid 

 mass projecting into the yolk ; they multiply and spread through the 

 yolk, ingesting it as they proceed, and a cavity appears in the interior 

 of the mass filled with disorganized cells. Eventually they reach its 



tnv 



end 



FIG. 140. Portions of two sagittal sections through developing eggs of 

 Homarus americanus. (Alter Herrick.j 



A, stage in which the endoderm cells form a solid mass. B, stage in which the endndiTiii tflls air 

 si>r>';iding through the yolk, deij, degenerate remains of more central cells; end, emloderm ; in/o, 

 cavity of invagination. 



surface, and here form an investing layer; thus secondary yolk 

 pyramids are not formed. 



Much the same process occurs in the prawn Palaemon ; but here 

 the endoclerm cells become detached -from one another and wander 

 through the yolk and eventually arrange themselves in an epithelial 

 layer outside it ; when they have reached this position they become 

 columnar but never attain the length of the endoderm cells in 

 Astacus. This kind of development seems to be general throughout 

 the Decapoda. 



In Lucifer, however, as we have seen above, Brooks asserts that 

 the segmentation of the egg is total, that a hollow blastula con- 

 sisting of relatively few cells is formed, and that an invagination takes 



