622 



COMPARATIVE ANATOMY 



Structures. If, for example, each rhombomere were metameric, it should 

 be connected with a visceral arch by such a cranial nerve as V, VII, IX or 

 X. At least the motor fibers of each nerve should have a center in a 

 single neuromere. It is found, however, that the mandibular branch of 

 the fifth nerve has its motor centers in two rhombomeres, the second and 

 third; that the centers of the seventh nerve lie in four rhombomeres, 

 the fourth to seventh inclusive, and that those of the ninth have their 

 nucleus in the sixth and seventh rhombomeres. Of the somatic motor 

 nerves, the relations of the abducens (VI) are even less clearly metameric. 



METEN, 



13 MM. SQUALUS EMBRYO -PARASAGITTAL SECTION. 



Fig. 516. — A diagram of the motor nerve relations of the hind-brain neuromeres, 

 based upon parasagittal sections of a 13-mm. squalus embryo, treated by the Ranson- 

 Cajal silver method. Similar nerve relations have been shown by Graper to occur in 

 bird and mammal embryos. The evidence does not strengthen the assumption of a 

 primary metameric correspondence between neuromere, nerves, myotomes and visceral 

 arches. 



Although the abducens innervates a single myotome, that of the third 

 head cavity, the motor nucleus stretches through three rhombomeres, 

 the fifth, sixth, and seventh. Since these relations occur both in elasmo- 

 branchs and in mammals, it may be a^ssumed that they occur in all other 

 vertebrates. While these confused nerve relations by no means prove 

 that the neuromeres are without metameric value, they are certainly not 

 the relations which would be expected in metameric structures. If 

 the relations of cranial nerves to muscles and visceral arches were originally 

 metameric, they have obviously been profoundly modified in the course of 

 phylogenesis. 



Doubt as to the value of neuromeres as criteria of metamerism also 

 arises out of the failure of students of neuromerism to agree on the number 

 of neuromeres in the head. This divergence of opinion is partly based 

 upon disagreement in regard to what structures may be counted as 



