SECT. 3] AND ORGANISATION 571 



notochord and somites behind this obstacle, and, on the other hand, 

 a cut between the organiser and the anterior region would exert 

 the same effect there. The influence evidently demands continuity 

 for its operation. 



It is important to distinguish the actual differentiation of this 

 middle period from the invisible determination which foreruns and 

 controls it. Spemann showed, for instance, that in the early neural 

 plate stage, before any sign was present of the optic vesicle, there 

 was a predetermination of the eye as a whole, and also the separate 

 areas for optic stalk, retina, and tapetal layer. Bulbar, ventricular, 

 atrial, and sinus substances are similarly determined before any 

 trace of the heart appears. Huxley proposed the term "chemo- 

 differentiation " for this invisible point of decision as to the fates of 

 cells *. After this point the organism is a chemical mosaic of qualita- 

 tively unUke regions, in which regeneration is impossible. "If a 

 gastrula be cut into two", he says, "each half forms only those 

 organs which it would have formed as part of the whole; again, if 

 a piece of the future brain region is cut out of the embryo in the 

 neural plate stage and then grafted back in the reversed position, 

 its different parts still produce the structures that would have been 

 produced in its normal position. Each chemically determined region 

 is separate and irreplaceable." And this state of affairs proceeds until 

 the onset of function, after which, with nervous control and the 

 activities of hormones in humoral channels, a third and quite dif- 

 ferent period comes into being. It is most important to note, mean- 

 while, that the point of chemodifferentiation is associated with 

 gastrulation. In the section on resistance and susceptibility a good 

 deal of evidence will be presented which tends to show that gastrula- 

 tion is one of the main critical points in development, points at 

 which a less degree of external interference is required than at any 

 other, in order to make development go wrong. 



The opportunities afforded by the tissue culture technique have 

 also been utilised in the study of the middle period of self-differentia- 

 tion or irreversible morphogenesis. Obviously the cultivation of an 

 explant of a certain region of embryonic tissue at different stages will 

 (or may) reveal whether it has been determined or not. This deter- 



* We do not know how far this process is really chemical. Decision would be easier 

 if we knew the rate at which, say, medullary plate ectoderm becomes specifically eye- 

 forming substance. Determination-rate is probably at least as important as Growth-rate 

 and Differentiation-rate. 



