MORPHOLOGY OF EYE MUSCLE NERVES 97 



tion with the posterior rectus muscle by the continuous outflow 

 of the protoplasm of cells in the somatic motor column of the 

 hindbrain. Its fibers are directly connected with the medullary 

 neuroblasts from which they arise, while its cellular elements 

 are partly medullary and possibly, in part, mesenchymatous. 

 From them develops the neurilemma of the differentiated nerve. 

 No evidence that they are genetically related to the neurofibrils 

 has been obtained. No sympathetic ganglion is associated with 

 the abducens anlage or with the adult nerve. 



5. What light does the study of histogenesis throw upon the ques- 

 tion of the homology of pre-otic and post-otic metameres? 



The demonstration of the similar histogenesis of the eye-mus- 

 cle and spinal somatic motor nerves creates a strong presump- 

 tion that pre-otic and post-otic divisions of the vertebrate body 

 are fundamentally alike. It has been found that in no essen- 

 tial respect does the histogenesis of eye-muscle and spinal so- 

 matic motor nerves differ. The dorsal chiasma of the troch- 

 learis and the lack of sympathetic connection with the abducens 

 in no way invalidates the comparison. The hypoglossus nerve 

 differs from the typical somatic motor nerves in precisely the 

 same way as the abducens, but its comparability with spinal 

 somatic motor nerves is unquestioned. In fact, if the relations 

 of somatic motor and sensory nerves in Amphioxus and Petro- 

 myzon may be regarded as primitive, those of the abducens and 

 of the hypoglossus are more primitive than those of other so- 

 matic motor nerves in Squalus. The absence of a sympathetic 

 is likewise a primitive character. The study of the histogenesis 

 of the eye-muscle nerves favors the prevalent conception of the 

 vertebrate head as once like the trunk. 



But the proof of the morphological similarity of segmental 

 nerves depends, not on evidence of histogenetic similarity alone 

 or central relations with the motor nidulus, but equally upon 

 the peripheral distribution. So that the question arises whether 

 or not the myotomes innervated by the eye-muscle nerves are 

 serially homologous with those innervated by spinal somatic 

 motor nerves. 



JOURNAL OJ MORPHOLOGY, VOL. 25, NO. 1 



