310 H. V. NEAL 



brief, are the reasons why the writer regards the primary brain 

 divisions as true neuromeres and not their secondarj^ subdi- 

 visions. Neuromere III, and not its secondary subdivisions, 

 'rhombomeres' 1 and 2, figures 1 and 2, therefore, is considered 

 a true neuromere. This conclusion is further supported by the 

 fact that 'neuromeres' II and III contain the motor niduH of the 

 nerves (oculomotor and trochlear) which innervate two succes- 

 sive myotomes (myotomes 1 and 2 of VanWijhe). Such func- 

 tional relationships of 'neuromeres' and mesomeres are cer- 

 tainly not accidental. But whether the metameric relations of 

 these two 'neuromeres' and myotomes are sufficient to establish 

 the metamerism of the more posterior 'neuromeres' or rhombo- 

 meres is doubtful in view of the motor nerve relations of the 

 latter. 



SUMMARY 



However doubtful the interpretation of the so-called neuro- 

 meres of vertebn^te embryos in other regions of the body, the 

 hindbrain neuromeres or 'rhombomeres' may be explained 

 neither as anlagen of adult organs nor as the passive results of 

 mechanical pressure of bending of the neural tube. A phylo- 

 genetic interpretation of them therefore appears to be not 

 impossible. 



None of the assumptions which underlie the phylogenetic in- 

 terpretation of rhombomeres, however, is undisputed. Not all 

 morphologists assume that the ancestors of Chordates were 

 metameric organisms. Moreover, neuromerism is not seen in 

 the central nervous system of Amphioxus. This fact suggests 

 that neuromerism is independently acquired by Chordates. 



In striking contrast with the mesomeric segmentation, the 

 neural segmentation is more conspicuous in the embryos of 

 higher Chordates than in the lower, more conspicuous in the 

 head than in the trunk. Analogous evidence, it will be recalled, 

 led to the abandonment of the vertebral theory of the skull. 



The rhombomeres may have arisen, as suggested by the writer 

 ('98), in adaptation to the branchiomeric segmentation. Doubt 

 still attaches to the serial homology of the rhombomeres with 

 other neural divisions (Neal, '98, p. 177). 



