576 ON INCREASE IN COMPLEXITY [pt. iii 



of the two main gradients is determined at or after fertilisation by 

 agencies external to the ovum. The other is determined in the un- 

 fertilised egg, but from an analogy with other forms, it is to be 

 expected that this too will be found to have been determined earlier 

 by agencies external to the oocyte (position of the oocyte in the 

 germinal epithelium, blood-supply, etc.)." Such is the general 

 scheme to which the recent researches in experimental embryology 

 have led. It is to be remarked that it includes in intimate association 

 with the working of the organiser the theory of metabolic gradients. 

 It will be necessary to treat this in some detail since it is based on 

 biochemical conceptions, and in some cases biochemical experiments. 

 But, before doing so, some further space must be devoted to the 

 organiser. 



What determines the direction of the differentiation induced by 

 an implanted organiser? It might determine for itself the direction 

 in which it would radiate its influence or this might be a function 

 of the host. Geinitz's experiments have shown that the latter alter- 

 native is the more probable one. At exactly what stage the cells 

 of the dorsal lip acquire their organising power is naturally very 

 difficult to ascertain, and has not yet been ascertained with accuracy. 

 Moskovski long ago found that, if the grey crescent in the frog's egg 

 was injured by pricking with a hot needle or other means, no em- 

 bryonic anlage or organ-rudiments would be developed, although 

 gastrulation was not inhibited, and Spemann obtained precisely 

 similar results by eliminating the dorsal (i.e. grey crescent-containing) 

 blastomere at the 2-cell stage. The organiser must therefore originate 

 very early. But Spemann has so far not been able to get a trans- 

 planted piece of grey crescent to function as an organiser, although 

 Bautzmann reports success with parts of blastulae. 



Of second-grade organisers Spemann says, "The optic vesicle can 

 be regarded as the organiser of the lens. Yet the vesicle itself, as well 

 as any organising power which it may possess, is not evolved through 

 mere self-differentiation of an anlage of an early gastrula: on the 

 contrary, the differentiation of this particular anlage itself is deter- 

 mined by an external stimulus. We have seen that at an early stage 

 of gastrulation the presumptive eye-anlage can be replaced by pre- 

 sumptive epidermis and it would be possible to choose the implant 

 so that it should contain the presumptive lens area. In this case 

 the two ectodermal layers would exchange their roles — the stimulator 



