154 AN INTRODUCTION TO MODERN GENETICS 



attached to such a piece, hats are much more frequently regenerated, 

 though this process does not begin until a certain latent period has 

 elapsed. The natural conclusion is that new hat substance has been 

 produced under the influence of the nucleus 



The nuclear determination of the morphogenetic substances is most 

 conclusively demonstrated by transplantations between different 

 species. A piece of the stem of A. mediterranean without a nucleus, 

 may be transplanted on to the nucleus-containing rhizoid region of 

 A. Wettsteinii, and then amputated and allowed to regenerate. It forms 

 a Wettsteinii '"'hat." Thus the nucleus imposes its specific character- 

 istics, even its pattern characters, on to the cytoplasm. Hammerling 

 concludes that the cytoplasm in this species has no determinative 

 influence on the type of development, but he points out that no re- 

 generation at all occurs unless the cytoplasm is in a reactive state; it 

 must, in our language, be competent to react to the organizing action 

 of the nuclear stuffs. 



This is perhaps the only case in which the production of morpho- 

 genetic substances has been shown to be under the direct control of the 

 nucleus. Unfortunately, we cannot take the conditions in a unicellular 

 organism such as Acetabularia as typical of those in Metazoa. It is, 

 however, very important to find a case in which pattern can be deter- 

 mined by a single substance. This is very difficult to understand unless 

 we suppose that the pattern is formed as a result of the interaction of 

 the substance with a spatially heterogeneous substrate. The importance 

 of the Acetabularia case is that it suggests that the factors which inter- 

 act to give rise to a pattern are chemical substances, just as are the 

 factors which merely stimulate histological changes and which we have 

 called evocators. 



9. Development as an Epigenetic Process 



One of the classical controversies in embryology was that between 

 the preformationists and the epigenisists. The former supposed that 

 all the characters of the adult organism were present in the fertilized 

 egg and only needed to be "unfolded" in some way during ontogenesis, 

 while the latter maintained that development involved the productioa 

 of something absolutely new, which arose from the interaction of the 

 original constituents of the zygote. After the account which has just 

 been given of the modern view of development in actual concrete cases 

 it is rather difficult to attach any very definite meaning to these old 

 theories. It is, surely, obvious that the fertilized egg contains consti- 



