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COMPARATIVE ANATOMY 



of the neural tube, distinguished by its affinity for specific nerve stains, 

 which reveal a deeply staining network within the cytoplasm of the 

 neuroblast. Subsequently, an amoeboid process extends from the cell 

 into the liquid-filled space between neural tube and myotome. This 

 space is narrow and connexion between neuroblast cell and myotome is 

 quickly effected. From the time of its first appearance, the nervous 

 character of this process is attested by its affinity for specific nerve stains. 

 The subsequent changes in this neurite involve its elongation along the 

 median surface of the myotome or its penetration into the mass of the 

 muscle fibers. Other neurites are added after the manner of the one 



MYOTOME- 



FiG. 459. — A camera drawing of a portion of a cross section of a 6 mm. elasmobranch 

 embryo showing a Rohon- Beard "giant" ganglion cell, stained by the vom Rath 

 method. In the specimen represented the body of the Rohon-Beard cell is located in 

 the dorsal wall of the spinal cord and its neuraxon process extends to the level of the 

 myotome. Periferally the neuraxon terminates in numerous amoeboid processes. 

 Sectioned material provides no better evidence than this of the correctness of the process 

 theory of Kupffer. 



first formed, until the connexion between tube and myotome becomes a 

 bundle of naked neurites. Cells from the surrounding mesenchyma are 

 soon added to the nerve anlage. In Elasmobranchs, cells migrate from 

 the wall of the neural tube to form the neurolemma of the differentiated 

 nerve. Finally, medullary sheaths appear, apparently as a result of a 

 reaction between neurite and neurolemma. 



One of the early objections raised against the process theory was that 

 it left unexplained the problem of how nerves find their way to their 

 end-organs. This difficulty, however, does not appear to be serious in 

 the case of most somatic nerves, since these are separated by a minimal 

 distance from their end-organ, the myotome. The difficulty is more 

 serious for such a nerve as the trochlearis, the fibers of which grow from 

 the floor of the metencephalon, emerge from the roof of the isthmus, and 

 grow a long distance through the mesenchyma surrounding the brain, 

 until they reach the anlage of the superior oblique muscle. We have 

 today no theory which satisfactorily explains this circuitous route; but 

 such are the facts even if we cannot explain them. A similar phenomenon 



