FACTORS IX ONTOGENY 



279 



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

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

 definite status of outcome for each Ijlastomere was determined 

 after successive early cleavages. All these results seem to be 



Fig, 159. — A, Normal gastrula of sea urchin, Echinus microluberculatus; B. gastrula of 

 sea urchin, Sphd-rechinus granularis, from a Uthium 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 develo})ment. 

 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 j)hitei 

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

 his eight and sixteen small gastruhe, and thirty-two tiny bias- 

 tuhie from the separate blast omeres of the third, fourth, and 

 lifth cleavages respectively; also Zoya's medusa embryos from 



Fig. 160. — .\bnormal larval stages of the soa 

 urchin, Spharechinus granularis, produced 

 by heat. (After Driesch.) 



