372 JOHN LEWIS BREMER 



from the . immediate vicinity of the permanent abducens roots, 

 is a true recurrent branch of the abducens, as opposed to the 

 other scattered rootlets more immediately connected with the 

 hypoglossal nerve, which might be called anterior hypoglossal 

 roots. The significance of these transient roots as a whole is the 

 indication that the vagus, glossopharyngeal, and facial nerves 

 were originally provided with ventral roots in addition to their 

 dorsal and lateral ones, and that probably the abducens was 

 diverted to its present disconnected position by the migration 

 of the more ventral pre-otic musculature to the vicinity of the 

 eyeball. The recurrent branches of the abducens should be 

 correlated either with the migration forward of a postotic myo- 

 tome to form the external rectus muscle, or, as Neal suggests, 

 with the loss of the postotic myotome and the 'piracy' of the 

 nerve in acquiring attachment to pre-otic muscles. 



If we examine the descriptions and drawings of those authors 

 who have mentioned these recurrent branches, mostly in con- 

 nection with studies on the derivation of the eye muscles, the 

 impression is left that they are occasional variations, small and 

 of significance only in the comparative morphology of the verte- 

 brate head. NeaV describing Acanthias, says "that not all of 

 the nerve fibrils extend anteriorly toward the third somite (van 

 Wijhe's), but that in later stages of development, e.g., in embryos 

 with 70-80 somites, a nerve fibril is seen to pass from the posterior 

 root of the nerve in a posterior direction toward the myotome of 

 the sixth somite." Miss Platt^ speaks very briefly of the abdu- 

 cens, also in Acanthias, as being "at first distributed not only to 

 the walls of the third head cavity, but also to the general meso- 

 derm posterior to that cavity." Thus two investigators find 

 these recurrent branches in the same class of vertebrates at 

 different embryonic ages, the one 'at first,' the other 'in later 

 stages.' Scammon, in the Normentafeln of Acanthias, does not 

 mention these branches, and in the material used by Scammon 

 in this laboratory, I have found them in only one or two embryos, 



^ Piatt, 1891.2, p. 101. (Full data for references in foot notes are given in 

 bibliography.) 



2 Neal, 1898, p. 232. 



