ON THE NA TURE AND CA USES OF DIFFERENTIA TION 3 I 5 



for the second and third quartets form mesoblast in the polyclade, 

 but ectoblast in the annelid and gasteropod ! In the latter forms 

 the mesoblast lies in a single cell belonging to a fourth quartet of 

 which the other three cells form entoblast. This shows conclusively 

 that the relation of the part to the whole is of an exceedingly subtle 

 character, and that the nature of the individual blastomere depends, 

 not merely upon its geometrical position, but upon its physiological 

 relation to the inherited organization of which it forms a part. 



Meanwhile, and subsequently, however, facts were determined 

 that threw doubts on the hypothesis of cytoplasmic isotropy and 

 led Driesch to a profound modification of his views, and in a 

 measure rehabilitated the theory of cytoplasmic localization. Whit- 

 man, Morgan, and Driesch himself showed that the cytoplasm of 

 the echinoderm egg is not strictly isotropic, as Hertwig assumed ; 

 for the ovum possesses a polarity predetermined before cleavage 

 begins, as proved by the fact that a group of small cells or micro- 

 meres always arises at a certain point which may be precisely located 

 before cleavage by reference to the eccentricity of the first cleavage- 

 nucleus. 1 Experiments on the eggs of other animals proved that the 

 predetermination of the cytoplasmic regions may be more extensive. 

 In the egg of the ctenophore, for example, Driesch and Morgan 

 ('95), confirming the earlier observations of Chun, proved that an 

 isolated blastomere of the two- or four-cell stage gives rise not to a 

 whole dwarf body, but to a half- or quarter-body, as Roux had 

 observed in the frog 2 (Fig. 135, AD). But, more than this, these 

 experimenters made the interesting discovery that if a part of the 

 cytoplasm of an unsegmented ctenophore-egg were removed, the 

 remainder gave rise to an incomplete larva, showing certain defects 

 wJiicJi represent tlie portions removed (Fig. 138, E, F). Again, 

 Crampton found that in case of the marine gasteropod Ilyanassa, 

 isolated blastomeres of two-cell or four-cell stages segmented exactly 

 as if forming part of an entire embryo and gave rise to fragments of 

 a larva, not to complete dwarfs, as in the echinoderm (Fig. 139). 



These results demonstrate that the ovum may show a high degree 

 of cytoplasmic localization and that in such cases cleavage may be in 

 fact a mosaic-work, as Roux maintained in case of the frog. But 

 they also show that the localization, and the resulting mosaic-like 

 cleavage, is not determined by specific differences in the nuclei ; for 

 in the ctenophore the fragment of an unsegmented egg, though con- 

 taining an entire nucleus, gives rise to a defective larva, and in Nereis 

 the nuclei may be shifted about at will without altering the develop- 



1 Cf. Fig. 77 . 



2 The larva is, however, not a strict partial one, since it makes an abortive attempt to 

 form the normal number of gastric pouches. 



