INDIVIDUATION— FORMATION OF PATTERN AND SHAPE 457 



inducing mesoderm. Spemann (193 1) pointed out that the inductive 

 capacities of the difFerent regions of the mesoderm were not all the same 

 even at the beginning of gastrulation. The presumptive anterior regions 

 tend, other things being equal, to induce more anterior parts of the 

 neural plate than do presumptive posterior regions. Shortly after this. 

 Mangold (1933) showed that if different regions from anterior to posterior 

 are cut out from the archenteron roof and implanted into young gastrulae 

 they show characteristic differences in the region of the nervous system 

 which they induce. It seemed then that one could consider the mesoderm 

 from the early gastrula stage onwards as having a fairly high degree of 

 regional specificity, so that the presumptive anterior part could be con- 

 sidered as a 'head organiser' and the presumptive posterior part as a 

 'trunk or tail organiser'. 



In agreement with this Hall (1937) showed that if the blastopore lip is 

 removed from a young gastrula and replaced by the lip from a consider- 

 ably older gastrula with a yolk plug, this presumptively posterior graft 

 failed to induce a head, the neural system of the resulting embryo having 

 a spinal character right to its anterior tip. Holtfreter (1936) has given a 

 map of these head organisers and trunk organisers (Fig. 10.3, p. 178). 

 Later authors have claimed that there are more than two regions— the 

 head and trunk— which behave independently of one another. Lehmann 

 (1945) and Dalcq (1947) have both argued that one must consider at least 

 the three main regions mentioned above, namely the forebrain, the 

 mid-brain, and hindbrain and the trunk regions. Nieuwkoop (1947) 

 presents evidence that in secondary induced embryonic axes the brain is 

 always fully formed up to a certain level, anterior to which it is altogether 

 absent, and on this basis he distinguished at least seven successive in- 

 dependent zones. 



The experimental results always made it clear that the regional speci- 

 ficity of an organiser was not something absolutely fixed in the sense that 

 the organiser could induce only one specific region of the neural plate 

 and nothing else. The results indicated at most a tendency for the in- 

 duction to have a certain regional character, but trunk organisers, for 

 instance, could sometimes induce more anterior parts and vice versa. 

 These disturbances in the simple picture can be to some extent accounted 

 for, at least in experiments in which the organisers are grafted into whole 

 embryos, by the supposition that the region of the host embryo in which 

 the graft lies exerts an influence on the regional character of the material 

 induced. However, a certain degree of latitude iu the specificity of a 

 particular region of the mesoderm is found even when it induces in 

 isolated pieces of ectoderm removed from the influence of any host 



