organisers: inducers of differentiation 



149 



words, meridional with reference to the host, and with its head 

 facing in the same direction as that of the host.^ 



The axis of the secondary embryo is determined by the direction 

 taken by the mass of material which is invaginated beneath the 

 surface in relation to the grafted organiser 

 fragment. It appears that the direction in 

 which this invagination occurs is deter- 

 mined in part by the orientation of the 

 grafted organiser,^ but in part also by the 

 activities of the host-tissues: the in- 

 vaginated mass tends to bend round 

 towards the animal pole of the host. This 

 has been discovered by grafting portions 

 of organiser with their original polarity 

 rotated 90° or 180° relative to that of the 

 host, so as to lie either transversely or 

 reversed. In almost all cases, some in- 

 fluence of the host is to be observed, 

 but the precise degree varies a great deal 

 in individual instances. In some cases, 

 the axis of an embryo derived from a re- 

 versed organiser may be completely de- 

 flected so as to coincide with the main host shown in this figure was 



, . , , induced by an organiser 



axis, but m other cases it may be almost grafted with reversed orien- 



precisely opposed to that of the host.^ ^^^^°^ ^"^^ the host; its 

 The orientating influence of the host is tli oT 'th^ p'r;'; e*! 

 greatest in the region surrounding the bryo; its anterior end is 

 blastopore, and least at the opposite pole. ""^ 1^ T""^ '"^ '^ "\^' 



^ /^^ ' ^t^ pv^iv.. ^j^^ jjgg transversely to the 



Un the other hand, what we may call the host. (From Spemann, 

 invaginating power of organisers varies, ^^''^^'- Euuumech. cxxm, 

 and is greater in organisers from old than 



in those from young gastrulae. Consequently, reversed orientation 

 of the secondary embryo is most often to be observed when 



^ Geinitz, 1925 a. 



2 The determination of the organiser to become invaginated is an instance of 

 what has been called " dynamic determination" (Vogt, 1923), leading to form- 

 changes which in turn result in the processes of gastrulation and neurulation (see 

 p. 26). The possible relation between dvnamic and chemo-differentiation is 

 discussed below (pp. 163, 250, 301). s Spemann, 1931. 



Fig. 70 

 The orientation of the 

 secondary embryo is depen- 

 dent partly on the polarity 

 of the host-tissues, and 

 partly on that of the grafted 

 organiser and the direction 

 in which it is implanted. 

 The secondary embryo 



