262 Edmund B. Wilson. 



to the greater number of cells produced, it gives rise to an embryo 

 that closes to form a morula- or blastula-like structure. The single 

 trochoblast of the 64-cell stage of Patella, finally, accomplishes no 

 more than a simple rounding out to a spherical form, without un- 

 dergoing further modification of its predestined development. 

 Each of the reactions in this series of forms must be considered 

 as a response to the change of conditions that results from a de- 

 struction of the relation of the part to the whole, and it seems to 

 me the different cases must be considered as differing not in kind 

 but in degree. In any one of these cases the inability to produce a 

 perfect larva is due, as I believe, not to absolute lack of regula- 

 tive capacity, but to lack of necessary material, which, as far as 

 the experiments show, the cell is not able to manufacture anew; 

 and the degree of regulative response may be considered, other 

 things equal, as inversely proportional to the degree of segrega- 

 tion that has taken place. Only, therefore, in a qualified sense, 

 and in a more or less limited degree, can the prospective value of 

 a cell be considered a function of its position.^ The sense in which 

 this saying applies to the upper group of four in an 8-cell stage of 

 Cerebratulus is far more limited than that in which it applies to 

 a lateral group of four from the same stage {cf. Zeleny) . As ap- 

 plied to an isolated primary trochoblast of Patella^ it becomes so 

 limited as to be largely deprived of its original meaning. The 

 same discrimination is necessary in considering the matter of dis- 

 tribution of potencies in the cleavage-pattern. When, for ex- 

 ample, Driesch asserts that "Furchungsmosaik brauch kein Mo- 

 saik der Potenzen zu bedeuten" ('99, p. 729, and elsewhere), he 

 is stating a fact that is incontestible as far as the 2- and 4-cell 

 stages of the sea-urchin or nemertine egg are concerned, and which 

 appears to apply to the medusa egg as far as the i6-cell stage; 

 but when in a later paper he advances to the statement, "Fur- 

 chungsmosaik ist kein Mosaik der Potenzen" ('02, i, p. 812), 

 an assertion is made that is contrary to the results of experiment, 

 not only on the molluscan egg from the beginning, but even on 

 the nemertine or echinoderm, as soon as the 8-ceIl stage is reached. 

 From the facts thus far determined the conclusion seems jus- 



iC/^. Wilson, '93, p. 610. 



