CLEAVAGE AND DIFFERENTIATION. 



33 



are present in many eggs different non-miscible substances 

 which may predispose cells during cleavage to essentially differ- 

 ent prospective values (micromeres and macromeres), and finally 

 definite substances are definitely localized in the eggs of many 



FIG. 12. The cross in Nereis and Crepidula. a, Nereis : the stippled cells are the inter- 

 mediate girdle cells (molluscan cross) excepting the posterior one (x 3 ) which corre- 

 sponds to the "tip cell " in the gasteropod. The trochoblasts lie at the margin of the 

 egg. (Wilson's Diagram II. B.) b, Crepidula: cross cells (intermediate girdle cells 

 of Nereis) are stippled. Apical and rosette cells unshaded as in a. Trochoblasts 

 around margin. c, Crepidula : shading as in b\ rosette cells and anterior trocho- 

 blasts divided. 



animals which permit one to recognize necessary relations to 

 certain early, firmly established organs. ... In certain cases 

 axial relations may be stamped upon eggs through the action 

 of external factors; in the majority of cases, however, especially 

 in eggs with complicated structure, this is not the case; the 

 organization is here performed in the unfertilized egg, that is, 



