PRIMORDIAL SOMA CELL (ECTODERM) 



ECTODERM CELLS A+B 



CHANGES SHAPE 





ECTODERM CELLS 

 •^ V"^ STOMODAEAL CELLS 



*^''M ENTODERM CELLS 

 MESODERM 



-^rrif — PRIMORDIAL 



■■'■^■- GERM CELL 



MESODERM 



ECTODERM 



Fig. 164. Distribution of presumptive organ regions (cell lineage) during cleavage 

 in Ascaris. (After Durken: Experimental Analysis of Development. New York, W. W. 

 Norton, based upon figures by Boveri and zur Strassen.) (A) Two-cell stage, showing 

 primordial soma cell and first stem cell. (B) Two ectodermal cells. A and B. Soma cell, 

 S,. is a mixture of mesoderm, stomodacum, and entoderm; second stem cell, P,. is a 

 mixture of mesoderm and germ-cell material. The symbolism used to designate the 

 various organ-forming substances is shown in (G). The progressive segregation into sep- 

 arate cells of the substances shown in cells S., and P, is given in (C-G). Cf. also fig. 6 IE. 



331 



