FACTORS IN ONTOGENY 



279 



the first cleavage of Synapta, and the cell lineage studies of zur 

 Strassen on the eggs of Ascaris in which it was shown that a 

 definite status of outcome for each blastomere was determined 

 after successive early cleavages. All these results seem to be 



A 



FIG. 159. A, Normal gastrula of sea urchin, Echinus microtuberculatus; B, gastrula of 

 sea urchin, S phcer echinus granularis, from a lithium culture. (After Herbst.) 



good evidence for preformation, that is, for a predetermination 

 of the role each part of the egg cell is to play in development. 

 Indeed, Wilson is convinced that an obvious structural differ- 

 entiation (bands, zones, delimited regions) can be seen in the 

 undeveloped eggs of numerous animals, a differentiation corre- 

 sponding to structural di- 

 vergence in development. 



On the other hand, nu- 

 merous results of experi- 

 ment speak just as loudly 

 against preformation or pre- 

 determination. Such are 

 Herlitzka's half-sized Triton 

 embryos from the two sepa- 

 rated first cleavage cells, 



Driesch's two half-sized and four quarter-sized sea-urchin plutei 

 from the cells of the first and second cleavages, respectively, 

 his eight and sixteen small gastrula 1 , and thirty-two tiny blas- 

 tulse from the separate blastomeres of the third, fourth, and 

 fifth cleavages respectively; also Zoya's medusa embryos from 



FIG. 160. Abnormal larval stages of the sea 

 urchin, Sphcerechinus granularis, produced 

 by heat. (After Driesch.) 



