92 H. V. NEAL 



* 



In view of the unanimous agreement of all investigators of 

 the histogenesis of the abducens that the nerve becomes second- 

 arily connected with its myotome, and in view of the entire 

 absence of evidence of primary plasmatic paths within which the 

 neuraxon might be differentiated; furthermore, in view of the 

 fact that it is possible to demonstrate the growing tip of the 

 abducens anlage as an amoeboid structure similar to those seen 

 in Harrison's cover-glass preparations, there appears little reason 

 to doubt that the abducens acquires a secondary connection with 

 its myotome. This inference is strengthened by Belogolowy's 

 discovery that some fibers of the abducens anlage of the chick 

 extend in their growth farther than the anlage of the posterior 

 rectus muscle and later atrophy. It seems easier to harmonize 

 such evidence with the hypothesis of the free outgrowth of nerve 

 fibers than with the hypothesis of primary connection of nerve 

 and muscle. Moreover, the fact that the nidulus of the ab- 

 ducens is about equidistant from post-otic and pre-otic myo- 

 tomes and that, in early stages of development, its fibers grow 

 in both directions — that is, both anteriad and posteriad — sug- 

 gests that the direction of growth is chemotropically determined. 

 The extension posteriad is, however, transient, the posterior 

 process is soon withdrawn, and the protoplasmic movement an- 

 teriad alone continues. 



h. What cells participate in the formation of the protoplasmic 

 connections? If the possibility of the development of the abdu- 

 cens by the progressive differentiation of plasmodesmatous paths 

 be excluded, the possibility remains that the abducens develops, 

 either by the differentiation of a chain of cells, or as the product 

 of protoplasmic movement of medullary neuroblasts. Dohrn 

 ('90 a, '91) alone, of all of the students of the histogenesis of 

 this nerve, has attempted to demonstrate the former mode of 

 histogenesis. The fact, however, that later ('07) be admitted 

 the inadequacy of the grounds upon which that opinion was 

 based makes it unnecessary to call attention to the fallacy of 

 his argument. Dohrn's premises were weak, not because he was 

 a poor observer — no man has done more to enrich our knowl- 

 edge of the histogenesis of nerves in selachians — but because his 



