MORPHOLOGY OF EYE MUSCLE NERVES 139 



the ancestral history of the head may yet be known and general 

 agreement among morphologists be attained. 



In both pre-otic and post-otic regions of the body the meso- 

 dermic corresponds numerically with the neuromeric segmenta- 

 tion. While the topographic alternation is not clear in the head 

 as in the trunk, the relations, both nervous and numerical, indi- 

 cate a primitive correspondence. 



The somatic motor column continues uninterruptedly from 

 post-otic into pre-otic regions. In both regions nervous connec- 

 tion with somitic musculature is effected similarly by the move- 

 ment of the protoplasm of the neuroblasts lying in that column. 

 The secondary connection of nerve and muscle in both regions 

 affords the possibility of the acquisition of new metameric rela- 

 tions such as appear in the case of the abducens nerve. The 

 relations of this post-otic nerve to pre-otic myotomes indicates 

 that no fundamental difference distinguishes the two regions. 



The somatic motor nerves acquire relations with somatic sen- 

 sory nerves and with sympathetic anlagen in the pre-otic region 

 in precisely the same manner as do spinal somatic motor nerves. 

 The misinterpretation of these relationships has long obscured 

 the perception of their true morphology and delayed the accept- 

 ance of the conclusion that head metameres are comparable with 

 those of the trunk. 



Typical pre-otic metameres, represented by the midbrain-ocu- 

 lomotor-premandibular and by the hindbrain-trochlear-mandibu- 

 lar segments, possess all of the essential components of typical 

 trunk metameres, namely, myotome, sclerotome, neuromere, so- 

 matic motor and somatic sensory nerves, and sympathetic an- 

 lagen. Their morphological comparability can be doubted only 

 by doubting facts which have been repeatedly confirmed. Neither 

 comparative anatomy nor embryology justify the speculation 

 that these elements are of exogenous, post-otic origin. 



Over against such evidence, we have differences between pre- 

 otic and post-otic regions such as would be expected in highly 

 differentiated regions. But the considerations advanced above 

 indicate that these differences are differences of detail and are 

 not fundamental. They would appeal more strongly as objec- 



