THE EPIGENETICS OF THE EMBRYONIC AXIS l8l 



organiser has a tendency to form some particular organs of the embryonic 

 mesoderm, it usually develops into, and induces, a larger region of the 

 embryo than it would have done if left in place. Moreover, the different 

 regionalities can influence one another. Tail organisers, grafted into the 

 head region of a host, often induce heads instead of tails. This is not be- 

 cause the competent ectoderm there has an inherent tendency to react 

 by forming a head; it is a consequence of the influence of the nearby host 

 head organiser which tends to force the small grafted fragment to take on 

 the character of the part of the embryo in which it Hes. Again, if a small 

 part of the organisation centre is excised and replaced in reversed orienta- 

 tion its surroundings may force it to conform with them, so that a normal 

 embryo results (Abercrombie 1950 in the chick, Waddington and Yao 

 1950 in Amphibia, see p. 458). 



The action of a graft in forming, together with what it succeeds in 

 inducing, a more or less complete organ, and the action of one organiser 

 on the regionality of another are both examples of a tendency by a 

 fragment of organiser to form a whole and complete unit — either a 

 complete organ or a complete embryo. Spemann at first tended to think 

 that this unit-forming tendency was an essential property of the organiser; 

 in fact, the very name 'organisation centre' which he gave his discovery 

 seems to imply something of that kind. However, Waddington and 

 Schmidt (1933) were able to show that this is not the case. The capacity 

 to perform an induction of some kind or another can be dissociated from, 

 any tendency to produce the missing parts of a complete unit, or to induce 

 any specific region of the embryo. The two aspects of organiser action 

 can be experimentally separated from one another, and one can have 

 grafts which induce but which cannot truly be said to organise. The first 

 clear demonstration of this arose during experiments on the organisers 

 of the chick embryo, and it is now time to turn to a consideration of the 

 epigenetic features of bird development. 



2. Birds 



The study of the epigenetic processes involved in avian development 

 (Review: Waddington 1952(7) was held up by technical difficulties greater 

 than those offered by the Amphibia. There were not only the usual 

 obstacles of small size, but the embryo is located under a hard shell and 

 viscous albumen, and on top of a fluid 'yoll^'- Early experimenters, such 

 as Hoadley, succeeded in cutting the blastoderm in half and following 

 the development of each part; and others, particularly Willier and his 

 students, cut out small fragments of the embryo and got them to develop 

 in isolation by placing them where they could obtain nourishment from 



