THE EPIGENETICS OF THE EMBRYONIC AXIS I77 



and to do this by means of a mechanism which operates during normal 

 development. Thus a piece of gastrula ectoderm can be made to differen- 

 tiate into neural tissue by placing it in the near neighbourhood of an 

 organiser. This remains a step of the utmost importance in the history of 

 embryology. It did not, of course, provide a fmal answer to all the prob- 

 lems of development. Some authors have been so zealous in emphasising 

 this (e.g. Weiss 1935, 1939, 1950/') that they have tended to suggest that the 

 whole concept can be dropped from our thinking, which is indeed to 

 throw the baby out with the bath water. What is called for is not a re- 

 jection of Spemann's ideas, but a further analysis and clarification of them. 

 The phenomena which he had revealed are certainly complex. Thus 

 'induction' has two different aspects, evocation and individuation, while 

 an essential role in the whole process is played by the 'competence' 

 of the reacting tissues. We shall have to discuss these concepts further 

 below. 



The next few years following the discovery of embryonic induction 

 were naturally spent in a general survey of the organiser's properties. 

 The extent of the organisation centre was examined by inserting small 

 fragments of one gastrula into the blastocoel cavity of another; the invag- 

 inating mesoderm of the host presses the graft up against the ectoderm 

 on the ventral side, and if it possesses any inducing power, a new embry- 

 onic axis appears there (Fig. 10.4, p. 180). It was found that the organiser 

 is at least as large as the region which will develop into the axial mesoderm, 

 (the notochord and somites); that is, the original grey crescent. But 

 its boundaries are somewhat vague, since the inducing power falls off 

 gradually from the centre of this region. In early stages, some degree 

 of inducing power has been shown to exist even in ventral regions (Dalcq 

 and Huang 1948, Dalcq 1950) ; and the capacity for induction is quite defin- 

 ite though weak, outside the axial mesoderm in late gastrulae, so that even 

 the lateral parts of the mesoderm can induce when planted into quite 

 young hosts (Waddington 193 6^). The organiser region is, in fact, a 

 'field system' in which the peripheral parts are dominated by the 

 centre. 



Fragments of early gastrulae which have the power to induce always 

 themselves develop into some mesodermal tissues, although they may 

 also form neural, and even endodermal tissues. This point was very well 

 investigated after Holtfreter worked out a salt solution in which embry- 

 onic amphibian tissues would survive and develop, using up their stores of 

 yolk as nutrients. He showed (1938^1) that isolated pieces of presumptive 

 mesoderm could develop into very many different tissues, although the 

 region from which a given organ was obtained was roughly centred on 



