THE MUTATED GENE 179 



As an example of this type, Twitty's recent work (1936) on 

 newts may be mentioned. He transplanted the parts of the 

 embryo that produce the pigment cells reciprocally between 

 different species of Triturus, which are characterized by a different 

 arrangement of their pigment cells. The transplants kept the 

 arrangement of the species from which they came. An example 

 of the interactions of both types of differentiation is the well- 

 known experiments of Spemann, Schott6, Holtfreter, et al. 

 The differentiation of the region around the amphibian mouth is 

 of the inductive type. If the inductor is taken from a Triton, 

 and the tissue to be induced from a frog, the inductor induces 

 differentiations of mouth organs, but the specific type of organs 

 (horn teeth, etc.) is controlled by the genes in the induced tissue. 



As far as experimental information goes, we may assume that 

 in vertebrates the inductor type is more frequent in development ; 

 in insects, however, self-differentiation is more frequently met 

 with, though not so exclusively as was believed a few years ago. 

 (Embryological work of Seidel and students; inferences from 

 genetic work as reported here.) 



Thus far, we have considered only such gene-controlled reac- 

 tions within the cells as occur at the end of development. But 

 the majority of hereditary reactions occurring within one cell 

 are those occurring within the animal egg in its first stages of 

 development. In a later chapter, in which the problem of 

 pattern Avill be treated, we shall discuss the problems connected 

 with the early differentiation of the egg, and we shall mention 

 the cases in which genes are known to interfere in this process. 

 As yet there has been, in general, no genetic analysis of these 

 intracellular rearrangements of area with restricted determina- 

 tion (organ-forming stuffs, embryonic fields, etc.) which occurs 

 in the one-cell stage of the eggs of determinative type and con- 

 tinues through early development. The only point known is 

 that these processes are hereditary and specific and that they 

 may occur with different speeds in nearly related species. The 

 geneticist cannot doubt that such processes of distribution and 

 arrangement of cytoplasmic components occur under control of 

 the genes. The work of Haemmerling, which will be reported on 

 page 192, has proved such control at least for a complex plant cell, 

 and the author is in agreement with him when he extends his 

 conclusions to the developing egg. Experimental embryologists, 



