300 H. V. NEAL 



in Amphioxus, all of the somites give rise to permanent myo- 

 tomes (Koltzoff, '02; Neal, '17). An homologous metamerism 

 of the mesoderm is found in Elasmobranch embryos (VanWijhe, 

 '82, and many others), but some of the somites in the head re- 

 gion are greatly reduced and two or three fail to differentiate 

 permanent myotomes. Cephalic somites are even less distinct 

 in Amphibian embryo (Piatt, '94, '97). In Amniote embryos 

 the only remnants of the pre-otic mesodermic segmentation are 

 seen in the 'head-cavities' which give rise to the eye muscles. 

 Such evidence accords perfectly with the assumption that the 

 mesodermic metamerism was characteristic of the ancestors of 

 Chordates. The contrast with the neuromeric segmentation 

 makes it difficult to believe that it represents a similar ancestral 

 metamerism. That morphologists will consider this difficulty 

 removed by the denial that the Leptocardii and Cyclostomes are 

 primitive types of Chordates is doubtful, for the majority of 

 morphologists believe that Amphioxus is the contemporaneous 

 form which most closely approximates the appearance of the 

 ancestors of Vertebrates. The absence of neuromeres in this 

 form, therefore, constitutes a serious objection to the hypothesis 

 that neuromeres are "the last remaining remnants of an ances- 

 tral metamarism." 



2. The striking structural and morphological differences be- 

 tween the so-called neuromeres in the different regions of the 

 central nervous system are not in accord with the supposition 

 that they represent a series of homologous segments. On the 

 basis of their structure, there is not the slightest evidence that 

 the neuromeres of the spinal cord ('myelomeres' of McClure, 

 '90) are other than the passive result of the mechanical pressure 

 of the adjacent mesodermic somites. They are limited to the 

 ventrolateral wall of the neural tube and they appear and dis- 

 appear in correlation with the appearance and disappearance of 

 the rounded contour of the adjacent somites. They are least 

 pronounced in the region of the neck where the somites are least 

 developed. The contrast with the ' rhomb omeres' is striking 

 and significant. No simple mechanical interpretation of the 

 latter is possible. For the rhombomeres are not only local ex- 



