MOSAIC THEORY 417 



by Hertwig is likely to be seen. The theory may find support 

 in the experiments of Morgan and Driesch on Ctenophore ova, 

 where a defect in the cytoplasm (not involving the nucleus) 

 is often followed by a modified cleavage and a defective embryo, 

 as if the architecture had been seriously injured ; but it may be 

 opposed by Delage's experiments on merogony, where a small 

 (and non-nucleated) fragment of a sea-urchin's egg may be 

 fertilised and give rise to a complete larva. In some cases like 

 the last it seems impossible to maintain that different parts 

 of the egg are predetermined in relation to particular structures, 

 and the same conclusion is suggested by Wilson's experiments 

 on the lancelet ovum, where an isolated blastomere of the 

 four-cell stage develops into a complete larva. In other cases, 

 however, it seems as if the egg had a fixed and set architecture, 

 which cannot be damaged without affecting the embryo. In 

 certain cases there is proof that the egg contains pre-formed, 

 and even pre-localised, organ-forming substances, and the re- 

 moval of a small part may be followed by the absence of a 

 definite structure, should development go on. In some cases it 

 seems clear that the old view of the ovum as homogeneous and 

 isotropic must give way before experimental evidence of hetero- 

 geneity. 



The " Preformation " mainly Nuclear. — But the researches 

 of Kolliker, Strasburger, Hertwig, and others led to a transfer- 

 ence of attention from the cytoplasm of the germ-cell to the 

 nucleus. From the importance of the nucleus in metabolism, in 

 the regeneration of Protozoon fragments, in maturation, in fertili- 

 sation, and in cleavage, it was argued — most forcibly, perhaps, by 

 Weismann — that the nucleus must be the bearer of the heritable 

 qualities. Meanwhile, many were recognising the value of 

 Nageli's conception (1884) of a specific idioplasm — a complex sub- 

 stance which, in its molecular organisation and in the metabolism 

 it induces, is different for each species. Weismann developed 

 this in his theory of the germ-plasm, which he regarded as wholly 



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