464 PRINCIPLES OF EMBRYOLOGY 



those responsible for the initial forebrain induction and the later trans- 

 forming action. If the whole regional determination is to be attributed 

 to quantitative variations in so few substances the responsibihty for most 

 of the details of the structure of the neural system must be attributed to 

 processes of self-individuation; that is to say, one must suppose that the 

 process of induction gives to a particular region only a general specifica- 

 tion to form forebrain or midbrain or some other part, and that the 

 details of the structure of the organ are elaborated by processes going on 

 within the reacting material. 



As Lehmann (i948/>) has pointed out, a self-individuating piece of tissue 

 seems usually to produce a certain range of structures quite completely, 

 and to lack the structures outside that range altogether, rather than to 

 cause the appearance of something which could be considered as a re- 

 duced edition of the whole. We have seen an example of this in Nieuw- 

 koop's fmding (p. 457) that an induced embryo which contains the trunk 

 will also contain all levels of the brain up to some point at which it stops, 

 the more anterior levels being completely absent. This can be interpreted 

 as indicating, not that there are separate different inducing substances for 

 each level which may or may not be present, but that a given mass of 

 induced neural tissue moulds itself into a neural system which is fully 

 formed as far as it goes and misses out the other parts entirely (Fig. 20.27). 

 The occurrence of self-individuation in the mesoderm as well as in 

 the neural system has been very strikingly demonstrated by Holtfreter 

 (1943 1)). He cut out the blastopore Up region from a young gastrula of 

 Triton and treated it with alkahne solution. The tissue then becomes 

 disaggregated and the cells fall apart. They can be thoroughly stirred 

 round so that their original arrangement is quite lost (as can be demon- 

 strated by mixing together cells from a vitally stained with those from 

 an unstained embryo). The loose cells can then be caused to aggregate 

 again by placing them in acid saline. After re-aggregation is complete 

 the mass can be implanted into the blastocoele of the host embryo; and 

 it is then found to differentiate into coherently arranged tissues and to 

 induce a neural axis which has well-defined regional properties. Thus the 

 re-aggregated organiser cells have produced within themselves a fairly 

 high degree of morphological organisation, clearly by a process of self- 

 individuation. 



We have so far considered the interaction between the sheet of inducing 

 mesoderm and the overlying ectoderm as though these two remained the 

 whole time in stationary contact with one another. It is, of course, clear 

 that this is not really the case. The patterns of regional differences within 

 the mesoderm and overlying neural plate arc coming into being at a time 



