EMBRYOLOGY 



509 



of the ovum is complete but 

 unequal, and 

 ESSEX" forms a hollow 

 blastula, which 

 invaginates to give rise to a 

 gastrula. The mesoderm arises 

 as two bands along the ventral 

 side formed by the division of 

 a pair of pole cells at the hind 

 end. In the crayfish the seg- 

 mentation is incomplete, but 

 of a different kind from that of 

 the chick and dogfish. The 

 nucleus divides till a number 

 of daughter nuclei are formed, 

 and these migrate to the 

 surface, where they are at 

 first embedded in a con- 

 tinuous sheet of protoplasm 

 which encloses a central mass 

 of yolk. Afterwards this proto- 

 plasm segments to form a one- 

 layered blastoderm enclosing 

 the yolk mass. Thus there 

 arises a sort of blastula which 



Fig. 387. — Stages in the develop- 

 ment of an earthworm. — After 

 Wilson. 



1. Stage of two blastomeres ; J>.c, polar 



bodies {not pole cells). 



2. Blastula ; M., primary mesoblast cell. 



3. Gastrula in ventral view ; Ec. y ectoderm 



or epiblast; En., endoderm or hypo- 

 blast ; M., mesoblast. 



4. Late gastrula in ventral view, showing 



the bands of cells known as " germ 

 bands"; M., mouth; m., primary 

 mesoblast cells (pole cells) ; fits. , meso- 

 derm bands ; N., nephridioblasts, 

 large cells derived from the ectoderm 

 which add to the bands of nephridial 

 cells as the embryo lengthens ; Nb., 

 neuroblasts, similar cells at the ends 

 of the bands of cells which form the 

 nerve cords ; w.c, nerve cords ; n^.c, 

 cells which will_form nephridia ; st., 

 stomodeeum. 



Ec 



St 



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