THE EPIGENETICS OF THE EMBRYONIC AXIS I93 



tissues within a self-individuating region depends primarily on the 

 production of inhibiting agents. He supposes that one region will develop 

 fastest, and will differentiate into some specific tissue. He suggests that 

 while doing so, it produces some substance which can diffuse into the 

 surroundings. This substance is supposed, in the first place, to bring the 

 original differentiation process to an end when a high enough concen- 

 tration is reached, and in the second to impede the tendency of the 

 neighbouring, more slowly developing, tissue to differentiate in the same 

 direction, and thus to swmg it over into some other course. The hypo- 

 thesis makes a pretty and coherent intellectual scheme, but in tliis simple 

 form suffers from the grave defect of neglecting the fact that all the 

 evidence suggests that embryonic tissues tend to induce the differentiation 

 of their like, rather than to suppress it. Thus, although Rose claims that 

 extracts of adult frog brain will suppress the formation of neural tissue in 

 the embryo, it is more relevant to normal development that young 

 neural tissue induces further similar neural tissue when placed in contact 

 with gastrula ectoderm (so-called 'homoiogenetic induction'). It is in fact 

 more plausible to suggest that differentiation is usually an 'autocatalytic 

 process', the substance produced by one type of differentiation tending to 

 encourage rather than to inhibit the same type of development. The re- 

 sults which Rose deduces from his postulated set of self-limiting reactions 

 would also follow equally from a system of self-reinforcing reactions 

 combined with competitive interactions (seep. 407). Inhibiting substances 

 of the kind postulated by Rose, may however play a part in regulating 

 the growth of the already differentiated tissues of the young adult. 



Rose conceives of the inhibiting substances which he postulates as 

 having an immunological specificity, and operating somewhat in the 

 manner of antibodies. It seems rather probable that developing tissues do 

 influence one another (and themselves) by the agency of substances of an 

 immunological character. The possibility has been discussed extensively 

 by Tyler (1947) and Weiss (1947); the latter author has some evidence 

 that adult organs may differentially stimulate the growth of homologous 

 embryonic ones — -just the opposite of what Rose suggests. An adequate 

 body of facts in this field is, however, still to seek. But there are indications 

 that a search for them may be rewarding. One may mention the observa- 

 tion of Ebert (1954) recorded on p. 215. 



4. The physiology of organiser action 



a. Natural and unnatural evocators 



This analysis of organiser action into two component parts was soon 

 exempHfied in quite another way. During the summer of 1932 both the 



