366 PRINCIPLES OF EMBRYOLOGY 



embryonic evocation ; but even so, it is not enough to carry our theories 

 on to firm ground. When a group of embryonic cells is induced to enter 

 on some particular path of differentiation, say towards the formation of 

 neural tissue, we know that the detailed character of the tissues produced 

 will be determined by the specific nature of the reacting cells. In a few 

 cases we can be certain that the specific character of the competent cells 

 is an expression of the genes they contain ; for instance when melanophores 

 are induced in genetically white or black axolotl tissue. And it is most 

 probable that in other cases, in which no analysis by genetic methods has 

 yet been made, most or all of the specific differences involved are to be 

 attributed to genes rather than to cytoplasmic factors. We have then good 

 grounds for supposing that when gastrula ectoderm is evocated to form 

 neural tissue, the set of gene-activities stimulated within it are different 

 from those which would be involved in the development of epidermis. 

 We have already described the basic information about the nature of the 

 evocation process (p. 206), but it needs to be examined again from the 

 point of view of gene activation. However, some of the theories about it 

 also involve the products, as well as the precursors, of gene activity, and 

 this discussion also will be postponed till chapter XIX. 



SUGGESTED READING 



Baltzer 1952&, Beale 1954, pp. 77-123, 148-163, Beerman 1952, King and Briggs 

 1953, Mather 1948a. 



