138 H. V. NEAL 



of comparative anatomy than does the assumption of more 

 numerous pre-oral segments. 



Assuming on such grounds the metameric value of those pri- 

 mary brain vesicles or neuromeres which correspond numerically 

 with the somites of Van Wijhe, more metameres are found ante- 

 rior to the ear than are admitted in the schemes of metamerism 

 of Ziegler and Brohmer, but fewer than in those of Johnston 

 and Belogolowy. In figure 76 are diagrammatically expressed 

 the primitive segmental relations based on the assumption of a 

 correspondence of primary neuromeres and Van Wijhe's somites. 

 That the abducens may be regarded as the somatic motor nerve 

 of a metamere posterior to the one whose myotome it innervates 

 seems indicated by its relations. Neuromeric relations therefore 

 afford an important clue to the primitive metameric relations 

 of the head. For this reason neuromeric segmentation may not be 

 disregarded in any attempt to define the metamerism of the head. 



7. Are these meta7neres serially hofnologous ivith those of the trunkf 



A summary of the evidence gathered by the last two gener- 

 ations of vertebrate morphologists convincingly demonstrates 

 that the vertebrate head possesses a metamerism comparable 

 with that of the trunk. First and most important, a true somitic 

 segmentation occurs in pre-otic and post-otic regions alike. An 

 identical segmentation characterizes selachian (Van Wijhe '82), 

 amphibian (Miss Piatt '97) and cyclostome fKoltzoff '01) em- 

 bryos. That is to say, these craniotes pass ontogenetically 

 through an acraniate stage, comparable to the adult form of 

 Amphioxus. 



The discovery by Koltzoff ('01-'02) that in embryos of Petro- 

 myzon a series of somites — exactly homologous with those dis- 

 covered by Van "Wijhe ('82) in selachian embryos — occurs, not 

 a single one of which disappears in ontogeny so that the mus- 

 cular metamerism is unbroken as in Amphioxus, appears to the 

 writer to be one of the most important made during the present 

 generation. The recent rehabilitation of Amphioxus as an an- 

 cestral type by Delsman ('13) seems to justify the hope that 



