WEIS'MANNISM AND OTHER THEORIES 407 



cells. In thus using the terms germ-plasm and soma-plasm (somato- 

 plasm) synonymously with chromatin and cytoplasm respectively, 

 Weismann's conception of the chromatin as the substance especially 

 important in heredity remains, although his theory of the dependence 

 of ontogenetic differentiation upon a sorting out of qualitatively differ- 

 ent units of this substance during development is no longer held. 



This use of the term germ-plasm is general among geneticists, who 

 are concerned with the problems of heredity, and may be distinguished 

 from that of certain students of the physiology of development, by whom 

 germ-plasm is regarded as "any protoplasm capable, under the proper 

 conditions, of undergoing regression, rejuvenescence, and reconstitution 

 into a new individual, organism, or part ' ' (Child 1915, p. 462) . From this 

 latter point of view the germ-plasm would be regarded as either the 

 complete protoplast capable as acting as so described, or, as Child is 

 inclined to believe, only an abstract idea merely a term standing for 

 heredity. 



Weismann's theory of the sorting out of hereditary units during onto- 

 genesis was abandoned not only because of irs inapplicability to the 

 results of certain experiments, but also because no support for it <^ould 

 be found in a direct study of the cell mechanism. Strasburger and other 

 investigators insisted strongly that so far as can be ascertained the 

 division of the chromatin at each somatic mitosis is exactly equational, 

 there being not the slightest indication of such a difference in the chro- 

 matin of the two daughter cells as might be expected were the divisions 

 qualitative (erbungleich) . To this Weismann had only to reply that 

 since the differentiation is a matter not of ids or of idants but of determi- 

 nants, the two nuclei would be visibly alike in spite of their qualitative 

 difference. Although certain cases have been described in which growth 

 is not equal in all of the chromosomes during the early stages of develop- 

 ment, and although the two daughter nuclei may become differentiated 

 through unequal nutrition after their formation, as Strasburger suggested, 

 most biologists have adopted the view that all of the somatic nuclei are 

 qualitatively alike in their chromatin content so far as its hereditary 

 powers are concerned. They have thus followed de Vries (1889) in 

 holding that factors for all of the hereditary characters are present in 

 all of the somatic cells, a conclusion strongly supported by the facts of 

 regeneration. The ontogenetic differentiation of the cells which mani- 

 fests itself largely in cytoplasmic changes, as well as the relative regen- 

 erative powers which these cells possess, are attributed for the most part 

 to physiological causes, the latter in large measure determining what 

 hereditary capabilities of the various cells shall come to expression. 

 The distinction between the view of Weismann and that of more recent 

 investigators is made clear in the two diagrams of Fig. 159, which have 

 been copied from Conklin (1919-1920). 



