THE EPIGENETICS OF THE EMBRYONIC AXIS 



175 



only the vegetative half developed; thus the crucial region is in the dorsal 

 vegetative quadrant v^hich contains the blastopore. The next step was to 

 graft a fragment from the blastopore region into another location in the 

 egg. It was found to develop, whatever its new position, into part of an 

 embryonic axis (Figs. lo.i, 10.2). Grafts from the presumptive ectoderm, 



Figure io.i 



An organiser graft by the 'Einsteck' technique. A piece of tissue from the 

 neighbourhood of the blastopore of one gastrula is inserted into the blasto- 

 coel of a second gastrula. The movements of gastrulation press it against the 

 ventral ectoderm of the host, in which it induces a secondary axis. The 

 diagrammatic view of this (below) shows that the organs may be normal 

 in shape, although formed partly from the graft (black) and partly from 

 host tissues. (From Holtfreter 195 1.) 



on the other hand, did not behave in any uniform manner, but developed 

 in accordance with their new surroundings. 



This not only showed that the blastopore region is the part which is 

 essential for the formation of an embryonic axis, but also suggested that 

 it acts as it were as a focus around which the whole egg is organised. 

 Spemann suggested that when a graft of ectoderm develops similarly 

 to its new surroundings, it is really its new relationship to the blastopore 

 which is determinative. A final proof of this came a few years later, when 

 Spemann and Hilde Mangold (1924) made grafts of the blastopore region 

 of gastrulae of Triton alpestris into gastrulae of another species of newt, 

 T. taeniatus. The tissues of the two species can be distinguished in stained 



