THE EPIGENETICS OF THE EMBRYONIC AXIS 185 



investigated by inserting small fragments between epiblast and endoderm, 

 in the way described above (Figs. 10.6, 10.7). As in the Amphibia, the in- 

 ducing region turns out to be about co-extensive with the presumptive 

 axial mesoderm. In birds, this is a fairly small part of the whole mesoderm, 

 since there is a large proportion of lateral mesoderm destined to migrate 



Figure 10.6 



Above, on the left, a chick blastoderm of the primitive streak stage ; at its 

 right anterior a piece of streak from another blastoderm has been grafted 

 between the epiblast and endoderm. On the right, part of a host (duck) 

 blastoderm, w^ith to the right a secondary head region induced by the anter- 

 ior half of a chick primitive streak. Below, a section showing the induced 

 brain (to the left) underlain by the graft, which has also developed some 

 neural tissue. (After Waddington and Schmidt 1933.) 



out to the sides of the blastoderm; this material probably has a wea 

 inducing capacity, and thus, in the fully grov^oi primitive streak stage, 

 grafts from the posterior half of that structure, consisting entirely o£ the 

 lateral mesoderm, may sometimes succeed in inducing, though they often 

 fail (Abercrombie 1954). There is also some organising capacity in the 

 regions of the blastoderm just lateral to the streak (Abercrombie and 

 Bellairs 1954). 



