280 DRIESCH'S THEORIES OF DEVELOPMENT V 



untenable. Quite apart from the fact that the complex archi- 

 tecture of the nucleus still demands an explanation an explana- 

 tion of the same kind, perhaps, which would at once involve us 

 in an infinite regress the facts which experiment has brought 

 out show conclusively that nuclear division is never a qualitative 

 process. In the other direction the hypothesis errs in attributing 

 a homogeneity, an isotropy to the cytoplasm, for the same 

 experiments have proved the existence in the ovum of definite 

 substances, necessarily concerned in the production of the 

 primary organs of the embryo. 



There is, however, no evidence to show that as imagined in 

 Weismann's, and to a certain extent in Roux's hypothesis there 

 is a separate morphological unit for each separately inheritable 

 character of the species ; such an idea would indeed seem to be 

 precluded by the ease with which, in gome cases at least, the 

 germ may be divided into parts, each of which is endowed with 

 the potentialities of the whole. 



And yet development is specific. How, then, is this mechanism 

 of inheritance to be conceived of ? It is to Hans Driesch that 

 we are indebted for an exhaustive attempt to think out the 

 whole problem. In his Analytische Theorie der organischen Ent- 

 wicklung Driesch starts with the facts with which we are 

 already acquainted, the similarity of the nuclei, the dissimilarity 

 of the cytoplasm in the several regions of the developing germ. 1 



The arrangement of these dissimilar substances determines 

 first of all an axis in the egg, an axis with unlike poles ; around 

 this axis the cytoplasm is radially symmetrical or isotropic, but 

 in the direction of the axis it is not ; or, as Boveri puts it, there 

 is a ' stratification ' of the substances of the egg at right angles 

 to the axis, the concentration of the animal substance decreasing 

 towards the vegetative pole, that of the vegetative substance 

 in the contrary direction. There are cases (Coelenterates, 

 Sponges, Ctenophora) in which this radial is the only sym- 

 metry; but in other types (Bilateralia) a third point may 

 be established by the disposition of some special substance (the 

 grey crescent in the Frog's egg or the yellow pigment in the 



1 It may be mentioned that in 1887 Plainer had already denied the 

 existence in development of any qualitative nuclear division. 



