96 H. V. NEAL 



ing as that seen in spinal somatic motor nerves. A section 

 through a root of the abducens is shown in figure 39 and the 

 appearances are seen to resemble closely those of early stages 

 of the development of ventral spinal roots. The evidence that 

 mesenchymatous cells attach themselves to the anlage is equivo- 

 cal but seems probable. Filatoff' s ('07) observation of a migra- 

 tion of mesenchymatous cells from the anlage of the posterior 

 rectus muscle is interpreted by the present writer as subsequent 

 to the acquisition of connection between brain and myotome 

 and as a part of the process of formation of the neurilemma. 

 Filatoff advances no evidence that these cells have a genetic 

 relation with the fibers of the definitive nerve. 



g. What is the fate of the cells of the abducens anlage? As there 

 is no sympathetic associated with the abducens in any verte- 

 brate, the cells of the abducens can have only one fate if they 

 persist in the adult nerve. In a non-ganglionic nerve they must 

 form the neurilemma. With this conclusion all investigators, 

 whatever their views regarding nerve histogenesis, agree. 



h. What is the histogenesis of the neurilemma of the abducens? 

 At the outset all of the cells of the abducens are peripheral in 

 relation to the fiber bundle. Penetration of cells into this bun- 

 dle is slow and has scarcely begun in a 25 mm. embryo. As 

 in the case of the oculomotor and trochlearis, penetration begins 

 in the region of the proximal roots, that is, in the oldest part 

 of the nerve anlage. By the time the embryo has reached a 

 length of 45 mm., however, cells have penetrated all parts of the 

 fibrillar bundle and have begun to assume the characteristic 

 form and relations of neurilemma cells. 



{. To what an extent do the emigrated medullary elements go to 

 form the sympathetic? The one essential respect in which the 

 abducens differs from the other somatic motor nerves described 

 is in the absence of the sympathetic in relation to the nerve 

 anlage. As in the case of the hypoglossus, however, this feature 

 appears correlated with the absence of connection with a gan- 

 glionic nerve, a correlation emphasized by Hoffmann ('00). 



j. Summary of the histogenesis of the abducens. Like typical 

 somatic motor nerves, the abducens acquires secondary connec- 



