I. NEURULATION 121 



transplantations of a lateral half of the organization centre of 

 one embryo into the ventral side of another. One edge of the 

 graft, therefore, consisted of mid-dorsal material which would 

 normally have formed a notochord, and the rest consisted of 

 prospective somites (cf. Fig. 35). The result of the experiment 

 depended on the age of the gastrula from which the graft was 

 taken. If it was taken from an older gastrula, its originally 

 median part formed a notochord, whereas the remainder pro- 

 duced a single series of somites. Both areas, therefore, differ- 

 entiated according to their original potencies (PL VIII, b) *. 

 Grafts taken from an early gastrula, however, produced a noto- 

 chord flanked on both sides by somites (PL Villa) *. This shows 

 that "regulation" took place here, which made possible the 

 development of a symmetrical archenteron roof from one half 

 of the organiser, and led to a deviation in the differentiation of 

 certain regions from their normal fate. In both cases, the host's 

 mesoderm supplied the missing parts of the archenteron roof. 

 The latter effect was also seen in another experiment, in which 

 the graft formed only a neural primordium, whereas the host 

 itself produced notochord and somites (Raven, 1938). The most 

 important aspect of these facts is that, whatever the shares 

 of graft and host in the composition of the organs may be, yet 

 in many cases a more or less complete secondary embryo is 

 formed. The tissues of graft and host supplement each other; 

 one produces what the other cannot supply. There is endless 

 variety, therefore, in the ways in which the secondary embryo 

 may arise from the co-operation of graft and host. The use 

 to which the cell material is put varies from case to case, and 

 its origin from graft or host does not determine its fate. The 

 organising process is not restricted to the organiser. From 

 there, it extends into the surrounding tissues as well, and 

 unites graft and host tissues into one integrated system. The 

 differentiation of each cell depends only on its position in this 

 system, and not on its origin. The best way to formulate these 

 facts seems to be the statement that, starting from the or- 

 ganiser, an organisation-field arises. 



The organisation-field is a system within which a number of 

 developmental factors are distributed in such a way that the 



Facing page 113. 



