organisers: inducers of differentiation 191 



§9 



In reviewing the various aspects of dependent differentiation it 

 is clear that the organiser phenomena occupy a special place. The 

 part which organisers play is of supreme importance. From the 

 theoretical point of view, they present a biological property of the 

 first order, and had Roux known of their existence he w^ould un- 

 doubtedly have classified them among the ''complex components" 

 of development (see p. 9). 



However, the precise mode of action of organisers cannot be 

 understood except in relation to the properties of gradient-fields : 

 this problem will be considered in some detail in Chaps, viii and ix. 

 Meanwhile, attention may be turned to the general result of the 

 presence in an embryo, such as an amphibian, of an organiser and 

 other structures, exerting effects of hetero- and homoio-genetic 

 induction, and some of them showing local regional differentiation. 

 The main result is that almost everywhere in the body formative 

 stimuli are found capable of inducing plastic tissues to undergo this 

 or that type of differentiation, according to their position. Nor- 

 mally, of course, the tissues cease to be plastic as soon as they have 

 undergone the inductive action of their organiser. But the exis- 

 tence, distribution, and local regional characters of the various in- 

 ductive influences in the amphibian embryo can be studied by 

 grafting portions of plastic early gastrula tissue into older hosts (at 

 the neurula stage), thanks to the fact that the inductive effects 

 persist for a longer time than is necessary for the normal determina- 

 tion of the embryo's own tissues. 



It has been found, using pieces of presumptive epidermis or 

 neural fold tissue as grafts, implanted into the dorso-lateral region 

 of neurulae, that the quality of the differentiations which the grafts 

 then undergo is dependent on their position in the host embryo. In 

 the head, grafts may differentiate into portions of brain wdth epi- 

 physis, nasal sacs, and eyes: in the gill region, into portions of 

 hind-brain : in the trunk region, into portions of spinal cord. The 

 grafts may be induced to form ear-vesicles, sense-organs, visceral 

 cartilages, and ganglion cells in the head; gills in the gill region; 

 fore-limbs in the fore-limb region ; pronephric tubules in the prone- 

 phric region, etc.^ (fig. 91). 



^ Holtfreter, 1933 b. 



