organisers: inducers of differentiation 151 



indications^) the organiser region is not divisible into right and 

 left portions possessing predetermined laterality; for a lateral 

 piece of primitive gut-roof, taken well to the left of the middle 





V 



C D 



Fig. 72 

 The "infective" properties of the organiser region in Urodeles. A, A piece of 

 presumptive ectoderm from the roof of a blastula of Triton crista tus is 'grafted into 

 the dorsal lip of the blastopore of a gastrula of T. alpestris, where it is plainly 

 visible on account of its light colour. B, The graft participates in the normal 

 gastrulation process of the host and becomes invaginated. C, When gastrulation 

 is completed, the embryo is cut open and the graft is found forming^'part of the 

 gut-roof in the mid-dorsal line, in the position of the notochord. D, The graft 

 is cut out and grafted a second time into a gastrula of T. taeniatus, where 

 it induces the formation of neural folds. (From Spemann and Geinitz, Arch. 

 Entzoniech. cix, 1927.) 



line, can induce the formation of a bilaterally symmetrical 

 secondary embryo when grafted^. One organiser region can thus 

 induce several embryos (see also p. 310). 



1 Goerttler, 1927. 2 Spemann, 1931. 



