90 H. V. NEAL 



demonstrated that sympathetic cells are wanting in the adult 

 trochlear, the possibility that the cell cluster associated with the 

 nerve in 25 mm. embryos forms a diffuse sympathetic along the 

 nerve in the adult is not excluded. As far as it goes, the evi- 

 dence favors the opinion that a transient sympathetic ganglion, 

 derived from the ramus ophthalmicus superficialis trigemini is 

 associated with the trochlear anlage as in the case of certain 

 anterior spinal nerves. There is no evidence whatever that 

 migrant medullary elements enter into this hypothetical tran- 

 sient sympathetic anlage. 



j. Summary of the histogenesis of the trochlear nerve. In the 

 manner of its growth and extension to form a secondary con- 

 nection with its myotome through the protoplasmic movement 

 of medullary neuroblasts; in the source of derivation of its neuri- 

 lemma; and in its relation to a ganglionic nerve and to a sympa- 

 thetic anlage, the histogenesis of the trochlear resembles in all 

 essentials that of spinal somatic motor nerves. To this extent 

 its serial homology with them is demonstrated. 



4. Does the histogenesis of the abducens resemble that of a somatic 



motor spinal nerve? 



a. Are protoplasmic connections betiveen hindbrain and myo- 

 tome 3 primary or secondary? No investigator has attempted to 

 support the Hensen hypothesis of the primary connection of 

 nerve and muscle on the basis of the histogenesis of the abducens 

 nerve. On the contrary all who have made a careful study of 

 its histogenesis— Dohrn ('90, '91), Neal ('98), Carpenter ('06), 

 and Belogolowy ('10) — agree that, at first, the abducens is not 

 connected with the posterior rectus muscle, but in its early 

 stages of development terminates freely in the mesenchj-ma at 

 the base of the brain. No evidence of the participation of plas- 

 modesmatous paths in the forward extension of the nerve has 

 been advanced. And, while no thorough investigation of its 

 development from the standpoint of the Hensen hypothesis has 

 been made, it does not seem likely that demonstrable plasmo- 

 desms would ha\'e remained unperceived. 



