88 MARIAN L. SHOREY 



As stated above, the necessity of a given primordium is well 

 established, but for each tissue we must still ask, Is some external 

 stimulus necessary to produce differentiation? If so, what is its 

 nature and source? 



For certain specific structures these questions may be answered, 

 at least in part, by experiments in regeneration or transplantation, 

 as has been done by Herbst in the study of the eye of the crab 

 and by Lewis in the study of the eye of the frog. But for tissues 

 which are not restricted to very limited areas, it is evident, that 

 neither of these methods can be effective. Nor can absolutely 

 crucial results be obtained by the removal, in forms in which 

 no regeneration occurs, of the primordium of some organ known 

 to be closely related in the adult to another; for such experiments 

 are always open to the objection that any abnormalities are purely 

 traumatic effects, an inhibitory substance being, perhaps, produced 

 by the wound. My own attempts to establish the necessity of 

 the presence of muscular end organs for the development of motor 

 nerves are a good example of this. From a most careful studj'^ of 

 all the conditions I am convinced that the effects of the wound 

 do not enter into the results, but I can give no conclusive evidence 

 that this is so. 



Therefore, when in 1907,^ Harrison published an account of the 

 development of nerve cells removed from the body of a frog and 

 placed in a drop of lymph, a distinct advance in method w^asmade, 

 at least theoretically. And I shall describe in this paper a few 

 out of a great number of experiments in which I applied this 

 method to a further study of the differentiation of neurones. From 

 the development of neuroblasts into nerve cells and fibers in lymph 

 Harrison concluded that they are self-differentiating, while from 

 the experiments in which I demonstrated that when the primordium 

 of given muscles is removed most of the motor cells which normally 

 innervate them fail to develop, I concluded that the motor neu- 

 rones are in some way dependent on the muscles for differentiation. 

 The only hypothesis by which these two lines of experiment can 



5 Harrison, R. G., '07. Observations on tiie Living Developing Nerve Fiber. 

 Ant. Rec, vol. 1. 



