GENES AND DEVELOPMENT 



151 



fragment fertilized by foreign sperm. Baltzer and Hadorn^ showed that 

 the bastard merogons die by reason of a necrosis of certain particular 

 regions of tissue. In the Triton palmatus 2 X Triton cristatus ^ hybrid, 

 the limiting tissue is the head mesenchyme, which dies at the neurula 

 stage. This is part of the organization centre, so here the incompati- 

 bility of nucleus and cytoplasm affects the organizer pattern. The 

 competence of tissues is also affected. Pieces of the merogon, other than 



.^ 





Fig. 74. Specific Differences expressed through Competence. — A newt larva 

 on to which Anuran ectoderm had been transplanted in the gastrula stage. A shows 

 that the ectoderm forms suckers, although no such organs occur in this position 

 in normal newt larvae. 6 is a section showing the Anuran material above and to 

 the right of the dotted line; it forms a harmonious part of the embryo, the pattern 

 of its development being largely controlled by the host organizer; but it retains 

 its own histological type. 



(From Holtfreter.) 



the head mesenchyme, may survive for a considerable time if trans- 

 planted into other surroundings in a normal embryo and one, at least, 

 of the tissues formed (epidermis), shows the specific characteristics 

 of th.e species from which its cytoplasm was derived. It is not known 

 whether the influence of the nuclear factors would be shown in organs 

 formed still later in development, as would be suggested by comparison 

 with the echinoderm experiments mentioned above. 



We may sum up by saying (i) that an organization centre induces 



the formation of new sorts of tissues by means of stimulating substances 



which are apparently the same in many different species ; (2) the type of 



tissue produced in response to the organizer is dependent on the 



^ Cf. Baltzer 1933, Hadorn 1935, I936, 1937. 



