SECT. 3] AND ORGANISATION 575 



plants), but that, on the other hand, the eye showed remarkable 

 capacities, producing all the constituents of the normal eye, 

 but not growing much in size. And with 6-day femurs, perfect 

 self-differentiation was produced in vitro by Fell & Robison. Most 

 interesting of all, Danchakov found, while studying the degree of 

 self-differentiation in various chick tissues, that the mesonephros 

 degenerated after a time in the grafts, just as it does in the 

 intact embryo. A commentary on the relations between tissues 

 acting as second-grade organisers and the tissues they influence is 

 seen in the observations of Champy, who found that epithelium and 

 connective tissue (from post-embryonic stages) in pure cultures by 

 themselves underwent dedifferentiation,iand reverted to an embryonic 

 condition. But if they were both present together in one culture the 

 differentiation was maintained, yet only so long as the connective 

 tissue constituent of such a culture was living. If it died the epi- 

 thelium dedifferentiated. In just the same way the kidney tissue of 

 the mouse grows alone as a sheet of undifferentiated Itissue, but can 

 be made to take on its usual character by the addition of connective- 

 tissue cells to the cultures (Drew). 



"It will be seen", says Huxley, "that the discovery of the organiser 

 and of the gradient coordinate system enables us for the first time 

 to give a coherent formal account (however imperfect in detail) of 

 the early stages in development. In so doing we sail clear of the 

 difficulty which has beset so many minds of understanding how 

 differentiation can be compatible with absence of qualitative nuclear 

 division. Loeb in his book The Organism as a Whole was driven to 

 assume that the Mendelian chromosomal genes were only responsible 

 for minor characters, the main course of development being deter- 

 mined by the ovum, which, owing to its assumed possession of organ- 

 forming materials, was to be regarded as the 'embryo-in-the-rough'. 

 It is now seen that the egg cannot be held to become the ' embryo- 

 in-the-rough ' until chemodifTerentiation has started. After this 

 moment specific organ-forming substances are all-important, but in 

 most unfertiHsed eggs they scarcely exist. The production of these 

 organ-forming substances depends upon the varying interaction of 

 organiser and genes in regions of various activity. The differential 

 which determines the variation of activity is the system of metabolic 

 gradients, which, although definitely organised, is very far from 

 constituting the egg an embryo, however much in the rough. One 



