MORPHOLOGY OF EYE MUSCLE NERVES 7 



conclusions are not so fully in accord, but all agree that the 

 ancestral vertebrate was a metameric acraniote, and there is 

 no suggestion that the meristic elements of the head have had 

 an exogenous source. 



2. Technical methods 



Considerable investigation upon the histogenesis of nervous 

 structures has been made upon material prepared by methods 

 unsuited to the special requirements of neurological investiga- 

 tion. To this may be attributed many of the fallacious infer- 

 ences that have delayed our knowledge of the true method 

 of nervous differentiation. Failure to use suitable neurological 

 methods is no longer excusable, however, since the advances 

 made by Cajal, Paton, Held and others. Tt is necessary for 

 one who attempts today to base morphological conclusions upon 

 the study of nerve histogenesis, to make sure by proper tech- 

 nique that he is dealing with real nervous structures and not 

 with pseudo-nerves. Not every cellular strand or spindle-shaped 

 cell, however closely associated with the central nervous system 

 or its derivatives, is necessarily a nervous anlage. Some cri- 

 terion by which a structure in the embryo may be identified as 

 nervous is needed. Hence the importance of stains which will 

 differentiate the neurofibrils from the time of their first appearance. 



Equally important in addition, is the use of a method which 

 will demonstrate cell boundaries and relations, especially in those 

 early stages when nervous connections between nerve center and 

 end-organ are estabhshed. Since no single method is known 

 which will effect both of these results with equal precision, 

 different methods for comparison and control of results appear 

 necessary. Trust in a single neurological method, however 

 highly perfected, may mislead as much as a less specific stain 

 has done. The importance of methods of preservation and 

 staining is evinced by the great divergence in the results, both 

 technical and theoretical, of Paton ('07) and Held ('09), each 

 of whom worked with a highly perfected technique. 



Therefore, while it may be granted that the older embryo- 

 logical methods are no longer adequate for the purposes of neu- 



