THE CRAYFISH AND ARTHROPODS IN GENERAL 215 



(Fig. 112, A). This division continues until there are a large 

 number of nuclei scattered about among the yolk spheres. Fi- 

 nally all of the nuclei migrate toward the periphery of the egg and 

 come to lie just beneath the surface (Fig. 112, B). Cell walls 

 then appear, dividing the egg into as many yolk pyramids as there 

 are nuclei (Fig. 112, C). The outer ends of these yolk pyramids 

 containing the nuclei are now cut off by cross walls, the result being 

 a single layer of cells, the blastoderm, surrounding the entire egg. 

 The side walls of the inner portion of the pyramids fuse, partially 

 restoring the original condition. 



FIG. 112. Three stages in the de- 

 velopment of the egg of the 

 crayfish. A, a few cleavage 

 nuclei in the egg; B, nuclei 

 arranged near the surface ; 

 C, egg divided into yolk 

 pyramids. (From Korschelt 

 and Heider. A and B, after 

 Morin ; C, after Reichenbach.) 



The embryology of the cray- 

 fish is so complex that a de- 

 tailed account will not be 

 attempted here. It will suffice to make a superficial examina- 

 tion of the developing embryo with the aid of Figures 112-115. 

 A. thickening of the blastoderm on one side of the egg is known 



