I INTRODUCTORY 19 



of yolk and protoplasm owing to the sinking of the former to 

 the lower, the rising of the latter to the upper side of the egg. 

 For though the egg thus comes to acquire a secondary structure 

 about an axis which is vertical, still the arrangement of the parts 

 of the supposed rudiments of the organs must have been dis- 

 turbed. Yet from such ova normal tadpoles are developed. 



It became necessary, therefore, to locate the self-differentiating 

 substance, the ' idioplasma ', mainly, at any rate, in the nucleus ; 

 and this idioplasma was imagined as composed of dissimilar 

 determinant units, each representative of some part or character 

 of the organism and arranged according to a plan or architecture 

 which corresponds in some way with the architecture in the 

 embryo of the organs represented. All that is necessary, then, 

 and all that happens, at least in the early stages of development, 

 is the gradual sundering of these units from one another by suc- 

 cessive qualitative divisions of the nucleus and their distribution 

 to the cytoplasm, where each determines the assumption by the 

 cell to which it is allocated of that character which it represents. 



Roux's ' Mosaik-theorie ' and Weismann's very similar but 

 more elaborate hypothesis of the constitution and behaviour of 

 the germ-plasm both frankly involve the belief that every 

 separately inheritable quality of the body has its own repre- 

 sentative in the germ, with the difference, however, that this 

 preformation, extended by Weismann to the adult characters, is 

 limited by Roux to those of the embryo. The renewed inquiry into 

 the nature and essence of development has thus simply resulted in 

 the resuscitation of the eighteenth-century doctrine of evolution, 

 though in a far more subtle form. Once again we find our- 

 selves face to face with the old alternative, Preformation or 

 Epigenesis ; and it is to the desire of solving this problem that 

 a very considerable proportion of modern experimental research 

 is attributable. Though much of this has been directed against 

 and been destructive of the 'Mosaik-theorie', which as far as 

 the nucleus is concerned has now been abandoned by Roux 

 himself, 1 renewed investigation has proved the existence in many 

 cases of definite and necessary organ-forming substances in the 

 cytoplasm, while the necessity for finding a causal explanation 



1 Roux, 1903. 

 C 2, 



