122 INDUCTION AND ORGANISATION 



formation of certain organ primordia in the field is thereby 

 determined. The fate of each element depends on its position in 

 the field. The field possesses the power of regulation. This 

 expresses itself in two phenomena. Firstly, it is not tied to 

 definite material elements, but instead it displays a certain 

 freedom as regards them. It can extend over the material, and 

 readjust itself; the field can be ''transposed". Secondly, its 

 size may vary according to circumstances. After the removal, 

 or addition, of material, it can adjust itself to the new con- 

 ditions. We shall now discuss some examples of these regulative 

 capacities. 



Early gastrulae of Triton can be halved, by transection or 

 constriction, in such a way that each part contains one half 

 of the organisation centre. Two complete embryos will be form- 

 ed in this case. Both may be slightly less well developed on the 

 side of the transection, but in all other respects they are 

 harmoniously built (Ruud and Spemann, 1922). Evidently an 

 entirely new organisation-field has arisen in each half. The 

 original field has been divided into two, and has, on a smaller 

 scale, reconstructed itself in each half. However, if the early 

 gastrula is halved frontally, so that its dorsal and ventral 

 halves are separated, the ventral half, which has no organisation 

 centre, will often produce only a "belly piece" lacking such 

 axial organs as neural tube, notochord and somites^). The 

 dorsal half, which contains the whole organisation centre, will 

 nearly always produce a harmoniously built embryo of half 

 the normal size (Fig. 44). Here, again, regulation takes place, 

 for in spite of the fact that the whole organisation centre is 

 present, the organisation-field is formed on a smaller scale, 

 adapted to the lesser proportions of the halved embryo. Dalcq 

 and Huang (1948) have repeated these experiments with a more 

 accurate technique. They have shown that the processes under- 

 lying this regulation are of a far more radical nature than 

 Ruud and Spemann suspected. An entirely new organisation 

 takes place in the halved germs, which disposes of the available 



1) According to recent work by Dalcq and Huang, howev^er, such a ventral 

 half will, if the ligature was made at the blastula stage, often form an embryo. 



