MORPHOLOGY OF EYE MUSCLE NERVES 89 



is by no means excluded that sympathetic cells are associated 

 with the adult trochlear nerve. This point needs investigation. 

 The important fact remains that a cell cluster, associated 

 with the embryonic trochlearis, has the same relations and 

 derivation as the sympathetic anlagen of the trunk and as the 

 ganglion of the oculomotor and that in this respect as in others 

 the nerve conforms in its histogenesis with that of a typical 

 somatic motor nerve. 



h. What is the histogenesis of the neurilemma of the trochlea!'? 

 More convincingly than any other nerve in the vertebrate body 

 the histogenesis of the trochlear anlage shows that a nerve may 

 become connected with its terminal organ without the slightest 

 indication of the participation of a single peripheral 'nerve cell' 

 or chain of nerve-forming cells. Dohrn ('07, p. 411) states that 

 it was the study of the histogenesis of the trochlearis which 

 converted him from a supporter of the cell-chain hypothesis to 

 one of the process theory of His. There appears in Pristiurus 

 (Dohrn) and in Squalus the decisive evidence which, in normal 

 embryos, effectually invalidates the cell-chain hypothesis in what- 

 ever modified form it may present itself. The evidence is wholly 

 convincing that the neurilemma cells of Scjualus have a second- 

 ary and exogenous derivation. Primarily, instead of a cell- 

 chain extending from nerve center to end-organ the trochlear 

 nerve appears as a fibrillar bundle. Into this fibrillar anlage 

 cells migrate from the neural tube and from the ramus super- 

 ficialis V. Some cellular additions may come from the adjacent 

 mesenchyma. These cells, generically independent of the fibers, 

 soon assume an elongated form and a relation to the nerve fibers 

 which marks their destination as neurilemma cells. 



i. To luhat an extent do the emigrant cells go to form the sym- 

 pathetic? The absence of any positive evidence of sympathetic 

 cells in the adult trochlearis would appear to afford presumptive 

 evidence against such a fate for any of the cells of the trochlear 

 anlage. But the fact that sympathetic anlagen disappear on- 

 togenetically in many spinal sor^iatic motor nerves makes the 

 supposition that the trochlear possesses a transient sympathetic 

 ganghon seem not unreasonable. Moreover, until it has been 



