PEIMARY NEUROMERES AND HEAD SEGMENTATION 353 



forebrain and midbrain at this stage does not show such segmen- 

 tation. In the primary neuromeres, he admitted that the seg- 

 mental arrangement of cells is" absent, and argued that the radial 

 cytological condition which later appears is due to the intraseg- 

 mental expansion of the walls. Thus the definite structure of the 

 later neuromeres he attempted to explain on purely mechanical 

 grounds. If, however, these definite and constant structural 

 features be merely mechanical effects, one wonders how he can 

 hope to substantiate the transitory, indefinite, and irregular 

 headings of the early stages as a primary neuromerism of phylo- 

 genetic importance. 



Study of the neuromeres of the later stages has also led to 

 great diversity of opinion. The neuromeres of the medulla are 

 definite structures with characteristic morphological features. 

 It is an open question whether or not they are homologous with 

 the divisions of the neural tube anterior and posterior to them. 

 The divisions of the spinal cord are undoubtedly formed by the 

 pressure of the adjacent mesodermic somites, and anterior to the 

 otic invagination the number and character of the somites are 

 far from established. It is in the anterior part of the neural 

 canal that the evidence is most scanty and indefinite. Neal 

 ('18) argues that the primary brain vesicles are the true neuro- 

 meres of the region, and in the opinion of the writer his argument 

 is clear and comprehensive. If the central nervous system of the 

 primitive vertebrate were segmented, with the enlargement of 

 the brain there would be an enlargement of the segments. The 

 brain segments enlarge laterally and dorsoventrally, and it seems 

 only natural that they should enlarge anteroposteriorly. It 

 certainly is as reasonable to suppose that individual segments 

 would expand anteroposteriorly as to account for the elongation 

 of the brain by fusion of segments and the backward migration of 

 the cephalic region with the concomitant incorporation of addi- 

 tional segments in the brain. While gill clefts, visceral arches, 

 and epibranchial organs are cenogenetic, still they may be seg- 

 mental, being predetermined in position by nerves, blood vessels, 

 septa, and other segmental structures of the invertebrate. The 

 later brain is highly developed, with great specialization of parts, 



