84 H. V. NEAL 



the evidence that "the fibrous root of the nerve cannot be com- 

 posed of prolongations from the distal cells (derived from the R. 

 ophth, sup. V), because the fibrous or proximal part of the troch- 

 learis arises before the distal or cellular part; further, the fibrous 

 root is the thickest as it comes from the brain, becoming grad- 

 ually attenuated as it proceeds into the mesoderm. " Belogolowy 

 ('10a) likewise finds the trochlear of the chick fibrillar from the 

 time of its first appearance. There appears no good reason to 

 doubt that the trochlear anlage is not only 'fibrillar' but 'neuro- 

 fibrillar' in the Apathy sense of the word. 



d. Is a neuraxon of the trochlear anlage multicellular in origin or 

 the process of a single 7nedullary cell? While it is practically im- 

 possible for any particular neuraxon of any nerve to give an 

 answer to this question upon the basis of direct observation (and 

 for the trochlear nerve the difficulty is increased by the extended 

 intra-medullary course of the fibers of the nerve) it is easy to dis- 

 prove the assertion that the trochlearis anlage in its peripheral 

 stretch is the product of the fusion of a chain of 'nerve' cells, 

 since in Squalus, as in Pristiurus (Dohrn '07), the nerve anlage 

 may be traced as a bundle of fibers, devoid of cells or nuclei, from 

 its point of emergence from the brain to its terminal organ. If 

 such evidence could convert Dohrn ('07) from the advocacy of 

 the cell-chain hypothesis of neurogenesis which on the basis of 

 evidence partly derived from the study of the same nerve he had 

 defended for years, it would seem that little argument would be 

 needed to convince unprejudiced students that the trochlear nerve 

 develops by the free outgrowth of medullary neuroblasts. It 

 appears unlikely that any student of neurogenesis will demand 

 that a neuraxon be traced from neuroblast to muscle fiber in order 

 to convince him of the applicability of the process theory to the 

 phenomena of histogenesis of the trochlear. So that, while in 

 the case of the trochlear nerve it is impossible to deal with indi- 

 vidual cells and fibers, the fact that, from the time of its first ap- 

 pearance as a fiber bundle emerging from the dorsal wall of the 

 brain, these fibers may be traced centrally to their connection with 

 a group of deeply stained neuroblasts lying in the somatic motor 

 column of the hindbrain, and that these are also the adult re- 



