RECURRENT BRANCHES OF ABDUCENS NERVE 385 



themselves tends further to approximate the postotic muscles 

 and nerve roots. Belogolowy/- as noted by Neal, found that 

 in Acanthias the roots of the abducens arise from neuromeres 

 next posterior to those from which they originate in chicks, thus 

 subjecting the caudal roots to a stronger postotic attraction, 

 represented by the occasional recurrent branch. The same is 

 very probably true in Heptanchus, but no material is available 

 to me for examination. The presence or absence of a recurrent 

 branch would depend upon a delicate balance between the time 

 of emergence of .the fibers from the brain and the degeneration 

 of the postotic somite, or perhaps its ability to produce muscle 

 fibers. 



It will be seen, then, that the presence of recurrent branches 

 of the abducens is correlated with the proximity of postotic 

 premuscle masses to the emerging roots, supplying apparently 

 an attraction in opposition to that of the eye-muscle group. 

 Probably the degree of differentiation of the various muscles 

 concerned plays a large role in the force of these attractions, 

 independent of their actual proximity, but our lack of knowledge 

 of the minute details of early muscle differentiation, or of possible 

 difference between muscles developed from definite somites and 

 those, like the eye muscles and the branchial musculature, arising 

 from mesenchymal condensations, does not permit us to be 

 critical in this respect. The three types of distribution of the 

 recurrent branches, to the lateral head musculature, to the 

 branchial muscles accompanying the vagus nerve, and to the 

 dorsal side of the head, show further that the postotic influence 

 may be from different sources. The first two types have a com- 

 mon course as far as the line of the vagus and hypoglossal junc- 

 tion, and the majority of recurrent branches follow this course, 

 but end before reaching the vagus, so that it is impossible to tell 

 whether they would have continued laterally or curved to accom- 

 pany the vagus or hypoglossal nerves. In the first case the 

 muscle attraction would have been the most anterior somite, 

 in the latter the mesodermal condensation to form the branchial 

 muscles. The third type represents those recurrent branches 



12 Belogolowy, 1910, p. 279. 



