Ill DIFFERENTIATION 109 



of the animal is represented by a separate unit or deter- 

 minant in the germ, whether in the cytoplasm or in the 

 nucleus, and there is indeed evidence to show that certain 

 characters are caused not by the independent development 

 of any such determinant, but by the action upon one another 

 of parts already in existence. The importance of this concep- 

 tion is obvious. The initial structure of the germ need not 

 be unnecessarily complicated by imagining it to comprise an 

 enormously large number of such separate material factors or 

 determinants. Given a certain number to start with, the rest 

 will follow by the actions and interactions of these upon one 

 another. Such interactions may probably be of the nature, not 

 of grossly mechanical influences, but of responses to stimuli. 



Thus in the fish Fundulus the yolk-sac is deeply pig- 

 mented, the chromatophores being especially closely aggre- 

 gated around the blood-vessels. When the creature is de- 

 prived of oxygen the pigment disappears, and it is supposed 

 that under normal conditions the pigment-cells are chemo- 

 tactically attracted by the oxygen in the blood (Loeb). 



Again, Herbst has found that when the larvae of Sea- 

 urchins are placed in solutions of lithium salts or in sea-water 

 deprived of the sulph-ion, or devoid of the carbon-ion, the 

 skeleton of the pluteus is either distorted in the first two 

 cases, or absent in the last. The distortion takes the form of 

 a multiplication of the tri-radiate spicules and the arms are 

 correspondingly multiplied. In water devoid of C0 2 there 

 are no spicules and no arms. Herbst has therefore urged that 

 normally the outgrowth of the ciliated ring into the arms is 

 due to a stimulus thigmotropic perhaps exerted by the 

 tip of the spicule. The converse of this is seen later on when 

 the arms diminish in length as the calcium carbonate of the 

 pluteus skeleton is made use of by the developing urchin. 



A third case is the lens of the vertebrate eye, which it is 

 said depends for its formation upon a stimulus given by the 

 optic vesicle or optic cup to the overlying ectoderm. This 

 statement rests upon evidence brought forward by Spemann 

 and more particularly by Lewis and Le Cron, that when (in 

 the tadpole) the optic vesicle is experimentally injured or 



