296 H. V.'NEAL 



as to the number, that the morphological standing of neuro- 

 meres in front of the hindbrain is problematic. The futm-e 

 use of neuromeres as criteria of metamerism will therefore largely 

 depend upon the demonstration of the metameric value of the 

 hindbrain segments, concerning whose number there is very 

 little disagreement. For in this region there is no danger of 

 confusing the anlagen of adult organs with the ancestral nervous 

 segments. What, then, is the argument in favor of the morpho- 

 logical \'alue of neuromeres? 



The general argument in favor of the metameric importance of 

 neuromeres may be stated briefly as follows: 



1 . The ancestors of Chordates were metameric animals with 

 a metameric nervous system such as is seen in Annelids. 



2. In accordance with the fundamental law of biogenesis, this 

 ancestral metamerism or neuromerism would be expected to 

 manifest itself ontogenetically, and possibly transiently, in the 

 central nervous system of vertebrate embryos. 



3. Such a segmentation affecting all regions of the nervous 

 system has been described by investigators, notably Locy ('95) 

 and Hill ('00), as occurring in the embryos of many classes of 

 Chordates. 



4. In the region of the medulla the neuromeres or rhombo- 

 meres appear to have constant relationships, through the inter- 

 mediation of nerves, with the visceral arches. The meta- 

 merism of the latter would therefore seem to indicate a corre- 

 sponding metamerism of the neuromeres. 



None of these assertions, however, is beyond cavil; none is 

 undisputed. Not all morphologists conclude that the ancestors 

 of Chordates were metameric animals. The possibility of the 

 independent acquisition of metamerism by Chordates is by no 

 means excluded and the application of the biogenetic law in the 

 interpretation of neuromeres loses all significance, except upon 

 the assumption of a metameric ancestry of Chordates. A rela- 

 tively large number of investigators have questioned the results 

 of Locy and Hill both as to their accuracy and as to their inter- 

 pretation. The serial homology of the rhombomeres (connected 

 with the cranial nerves) with the neural segments anterior and 



