THE EPIGENETICS OF THE EMBRYONIC AXIS I79 



gastrulation, and is certainly no longer present by the end of it; organiser 

 grafts into old gastrulae (closed yolk-plug stage) no longer produce 

 inductions. During the stage when the gastrula ectoderm can react, it is 

 said to be competent (Waddington 1932), and the period when the re- 

 action is possible is the period of competence. These words are also used 

 of other tissues which, at later stages, become competent to react to the 

 organising stimuli, which, as we shall see, are exerted by the various 

 organs as they gradually develop (Chapter XII). 



Competence can be thought of as a state of unstable equilibrium; the 

 tissue is poised between two or more alternative paths of development, 

 and may follow one or the other according to the organiser stimuli 

 acting on it. In the case we are now discussing the most obvious alterna- 

 tives are between the epidermal development path or the neural one. 

 But actually there is a tliird; the presumptive ectoderm may be con- 

 verted into mesoderm. This may occur even when the organiser graft is 

 made merely by inserting a fragment into the blastocoel, but it is better 

 shown by grafting a small fragment of presumptive ectoderm into the 

 middle of the presumptive mesoderm just above the blastopore lip; it is 

 then found that the graft becomes invaginated along with the host meso- 

 derm, and takes part in the formation of the host's mesodermal organs 

 (Spemann and Geinitz 1927, Raven 1938). Further, if such a graft, after 

 being allowed to invaginate, is then removed and grafted into still another 

 gastrula, it has now become an organiser itself, and can perform an in- 

 duction in its new host. We shall return later when discussing the physio- 

 logy of induction, to this 'infectivity' of the organiser (p. 195). 



By the end of gastrulation, the action of the primary organiser is over. 

 The competent tissue has been definitely swung into one or other of its 

 possible types of development; it is now definitely started on the way to 

 becoming either neural tissue, or epidermis, or mesoderm. Within each 

 of these types, a good deal of latitude is still open to it; it is still not fmally 

 settled whether it will become brain or spinal colunm; skin or an ear 

 vesicle or a lens; muscle or mesenchyme or part of the urinary apparatus. 

 But the initial choice of path has been made. One step of development 

 has been, in the usual phrase, 'determined', and if the tissue is allowed to 

 develop at all, it will develop in accordance with that determination. 



We have so far discussed the action of the organiser in terms of its 

 effect on tissue differentiation, speaking of its results as the formation of 

 neural tissue or epidermis, etc. This is a simphfication of what actually 

 happens. The result of an organiser graft is often the production of an 

 induced organ, i.e. something in which the tissues are shaped into a more or 

 less definite structure and related to one another as they would be in a part 



