No. 2.] ORGANIZATION OF THE EGG OF UNIO. 269 



the entire Q.g^, but is, as has been shown for echinoderms 

 especially, the property of any nucleated part of sufficient size, 

 the mechanism cannot be />rm«rz/y due to structurally differ- 

 entiated germ-areas. Even if such areas can be shown to 

 exist in any unsegmented o.^^, it would be necessary to regard 

 this as a secondary effect of the primary mechanism upon the 

 unsegmented egg. 



It has not been shown for the ^.g^ of Unio that separated 

 nucleated parts of definite size are capable of typical develop- 

 ment ; but even if it should be shown that they are not, that 

 typical development is dependent on the integrity of the 

 original Q-ZZ^ ^ could not consider that as evidence in favor of 

 localized germ-areas ; for it has been clearly demonstrated, in 

 the case of the frog at least, that partial development of iso- 

 lated blastomeres may be due to certain physical properties of 

 the protoplasm. Whether we are studying the typical develop- 

 ment of a part of an 0.^,% or of the entire &gg, we are con- 

 fronted with the same problem ; and all that the facts of typical 

 development of a part teach us is, that neither the nuclei nor 

 yet the protoplasm of early cleavage stages are unchange- 

 ably specified. Are these then the "positive results " that we 

 are led by Driesch to expect.^ So far as positive results are 

 concerned the entire problem still faces us, and the negative 

 conclusions were clearly seen by some naturalists as the result 

 of interpretation of facts of observation (" Nature's Experi- 

 ments "), long before the experiments were performed. 



But we may be told that complete and typical development 

 of a part, even from the moment of its separation, is no argu- 

 ment against the original localization of germinal areas, for the 

 typical regeneration of a fragment of Hydra does not alter the 

 fact that the parent animal had differentiated parts. But in 

 the absence of ocular demonstration of areas in the whole ^gg, 

 or of experimental proof of their occurrence, and in the face 

 of the fact that currents of rotation and translocation in the 

 protoplasm of the &gg in no wise affect the power of typical 

 development (Ziegler, '96), the theoretical necessities must be 

 hard that will drive us to such an hypothesis. Moreover, if 

 certain areas be removed from the ^gg, for instance, A and B 



