354 HORACE W. STUNKARD 



and ontogeny affords such fragmentary and inconclusive evi- 

 dence of phylogeny that neuromerism alone can hardly explain 

 the development of the head. The study of highly specialized 

 forms like the chick must appear of less importance than that of 

 more primitive forms, or at least of forms in which primitive 

 conditions persist. In this connection, Xeal ('18) has pointed 

 out that neuromeres are more conspicuous in the embryos of 

 higher, than they are in embryos of lower chordates, and this 

 would hardly be expected if they are vestiges of a primitive 

 neuromerism. 



In ontogeny, segmentation regularly appears first in the 

 mesoderm and the segmentation of the mesoderm is more con- 

 stant and regular than segmentation in other tissue. Segmenta- 

 tion of other tissue normally results from and is in correspondence 

 with segmentation of the mesoderm. Mesomeres are uniformly 

 present in the lower chordates, and to disregard mesodermal seg- 

 mentation is therefore to overlook an item of paramount im- 

 portance in any study of head segmentation. In the ancestral 

 vertebrate there was undoubtedly a correspondence of meso- 

 meres, neuromeres, cranial nerves, and branchial organs, and 

 all of these structures must be considered in an explanation of the 

 present lack of correspondence. 



The present study has shown that in Amblystoma and the chick 

 at least, the structures described by Locy and Hill as primary 

 segments cannot be regarded as metameric. Investigators have 

 repeatedly questioned the accuracy of the observations of Locy 

 and Hill, and a repetition of their work, using as far as possible 

 identical means of examination, has in the present case not only 

 failed to verify their observations, but disclosed a quite different 

 condition. The three morphological features upon which neuro- 

 merism can be based, marginal headings, external and internal 

 grooves, and cell arrangement, all fail to give evidence to con- 

 firm the primary neuromerism of Locy and Hill. Neal could 

 not confirm Locy's statements concerning Selachian embryos 

 and I have been unable to confirm Locy's observations on Amblys- 

 toma or Hill's on chick embryos. In my opinion, the so-called 

 'primary metamerism' is based upon incorrect observation and 



