16 H. V. NEAL 



2. Are muscle and nerve connected with each other ah initio? Are 



protoplasmic connections between myotome and 



tube primary or secondary? 



With the statement of this question we plunge at once into 

 the most controverted point in neurogenesis — the problem of 

 how nerves become connected with their terminal-organs. The 

 earlier students of head morphology avoided the details of nerve 

 histogenesis. Yet it is clear that the morphological resemblance 

 of nerves is determined quite as much by their cellular and cen- 

 tral connections as by the character of their terminal-organs. 

 In the adult elasmobranch and in the higher vertebrates these 

 relationships are frequently so greatly modified and complicated 

 that to unravel their intricacies requires the facts which the 

 ontogenesis of the lower vertebrates alone can give. Avoidance 

 of the details of histogenesis and reliance upon embryological 

 procedure unsuited to neurological investigation has led to the 

 inclusion of cell clusters or strands among cranial nerves with- 

 out proof of their nervous character and to erroneous inferences 

 regarding the phylogenesis of the head. Minot ('96) suggested 

 this weakness of earlier researches and the path of later investi- 

 gation when he wrote that 'Hhe attempt has been made to solve 

 the most difficult questions of the morphology of cranial nerves 

 without answering the inconvenient question of nerve fibers and 

 their sheaths." To-day the student of phylogenesis is no longer 

 able to shut his eyes to the necessity of a thorough investigation 

 of the genesis of nerves before drawing conclusions regarding 

 the past history of the vertebrate head. 



In the introduction to his masterly discussion of nerve histo- 

 genesis, Held ('09) states that notwithstanding the numerous 

 observations of the past fifty years and the multiplicity of 

 hypotheses, the question as to how nerves are formed in the 

 embryos of vertebrates is one of the most burning questions of 

 embryology : 



Many new methods have been applied to its solution and a surfeit 

 of new views have appeared, following one another in a short period 

 of time, but as yet no agreement has been reached regarding the true 

 principle of nerve development. This is due to a great extent to the 



