MORPHOLOGY OF EYE MUSCLE NERVES 143 



a transient sympathetic aniage. While the chiasma of the troch- 

 lear is an anomaly, it may be regarded as coenogenetic and its 

 existence does not invalidate the comparison of this metamere 

 with a trunk segment. The ramus mandibularis trigemini ap- 

 pears to be the splanchnic motor element of this metamere. 



The fourth metamere contains the third or hyoid myotome 

 and the fourth neuromere (second hindbrain neuromere). To 

 this segment may be assigned as the somatic sensory nerve the 

 major root of the trigeminal in part. Since no neural crest is 

 proliferated from this neuromere, however, this assignment must 

 be made with a question mark, although the major root of the 

 trigeminal is attached to this neuromere. The neuroblasts in 

 the somatic motor column of this neuromere do not produce a 

 nerve. The transient nerve seen in this region in chick embryos 

 (Belogolowy '10) may be the somatic motor nerve of this meta- 

 mere which has disappeared phylogenetically. The myotome 

 of the metamere, however, is innervated by the nerve of a post- 

 otic metamere, the abducens. An attempt has been made above 

 to explain this anomalous relationship which does not appear 

 to vitiate the comparison with a. trunk metamere. 



The fifth and last pre-otic metamere includes the fifth neuro- 

 mere and the fourth somite which is partly sub-otic, a position 

 to which it presumably owes the loss of its myotome. To the 

 degeneration of the myotome may be attributed the loss of the 

 somatic motor nerve of this metamere. No sympathetic aniage 

 develops in this segment and the somatic sensory components 

 are also lost. But the proliferation of the cells of the facialis 

 nerve from this neuromere justifies the inference that they once 

 have been present in this nerve. The loss of the myotome of 

 this and of the following somite, a loss in all probability due to 

 the enlargement of nerve ganglia and sense organ in this region, 

 tends to show that the preservation of the myotomes of the 

 first, second and third somites is due to their functional rela- 

 tion with the eye-ball. The eye muscles are the last remnants 

 of the lateral trunk musculature anterior to the ear. Their earlier 

 relations with post-otic myotomes are diagrammatically expressed 

 in figure 81. 



