The Reactions of the Vertebrate Embryo to Stimulation etc. 571 



are at first surrounded simply by the protoplasm out of which they 

 are ditferentiated , but gradually as they extend become continuous 

 with those lying inside of the cells, which are actiially imbedded 

 in the cytoplasm. In what manner it may be asked are the neuro- 

 fibrils actually differentiated ? Do they grow into the neuroblasts 

 as Apathy believes to be the case, or is their development centrifugai 

 as Held has sug-gested, so that they grow away from or out of 

 the cells and pass into the bridges, or does a third possibility exist 

 that they are laid down in situ within the protoplasm, and thus their in- 

 crease in length is the result of a progressive differentiation (etappen- 

 weise)? There does not seem to be any direct evidence in favor 

 of the ingrowth theory as first annunciated by Apathy, so that only 

 the two remaining possibilities bave to be considered. Held, on 

 what seems to be purely theoretical reasons, opposes the idea that 

 the fibrils are laid down in situ and grow by the addition at their 

 ends of freshly ditferentiated material. His objection does not seem 

 to be altogether valid, particularly in the case of the ventral roots 

 where the differentiation in the neuroblasts begins in the apical 

 processes, proceeds towards the opposite pole and is then continued 

 on beyond the appareut limits of the celi in the direction of the 

 centrai canal. If, with the results of this Observation in mind, the 

 attempt is made to explain the development of the fibrils as out- 

 growths from the neuroblast, the necessity arises of admitting that 

 within the celi is a point from which the process of neurofibrillation 

 spreads in opposite directious: on the one side towards the neuraxon 

 and on the other towards the opposite pole. 



By far the easiest solution of the problem seems to be to 

 suppose that the neurofibrils are differentiated in the protoplasm of 

 the cells, and in the plasmodesmata as the result of physiological 

 processes, either purely metabolic in character or initiated by incident 

 Stimuli transmitted from the periphery. The outgrowth theory is 

 most difficult of application in regard to the development of the ven- 

 tral roots, where the whole process of neurofibrillation is so evidently 

 from the exterual towards the internai layers. In the axons of cells, 

 which may be followed for some distauces, fibrils may frequently be 

 detected apparently lengthening as the processes themselves grow. 

 This appearance, however, does not at ali preclude the possibility 

 of the differentiation taking place in situ, advancing step by step 

 as the axon increases in length. 



As far as one may judge from the study of specimens prepared 



