394 CHARLES HILL, 



III. General Considerations. 



Identification of Neural Segments. 

 Neural Segments in Teleosts, Historical and Critical. 

 Neural Segments in the Chick, Historical and Critical. 

 Value of various segmental Criteria. 



a) Mesomeres. 



b) Branchiomeres. 



c) Neural Segments. 

 Summar3^ 



Bibliography. 

 Plates. 



I. Introductory. 



1. Review of Literature. 



That the Vertebrate head is composed of segments similar to 

 those that constitute the trunk, has gradually become a fixed point 

 in philosophical morphology. Although there is no dissent upon the 

 question of the segmental nature of the Vertebrate head, the question 

 of what constitutes the criteria of the segments, of what is their 

 number and their anterior limit, is still under controversy. There 

 has been a gradual change of front and the evidence has been shifted 

 from suturai joints (Oken, 1807; Goethe, 1820; Owen, 1832), to 

 cranial nerves (Huxley, 1859), to branchial clefts and cranial nerves 

 (Gegenbaur, 1878), head cavities (Balfour, 1878; Van Wijhe, 1882, 

 and others), and finally to segments of the neural tube (Béraneck, 

 1884; Orr, 1887; iAIcClure, 1860; Waters, 1892; Locy, 1895). 



The literature in all these lines has been reviewed somewhat 

 extensively and it seems necessary for the purpose of this paper to 

 give only a brief review of the more important results attained by 

 study of a) the Mesoderm of the Cephalic Region, and b) the Seg- 

 ments of the Encephalon. 



a) The Mesomeres of the Cephalic Region. 

 Balfour pointed out in 1878 that the mesoblastic somites are 

 present in the head in a modified form. To them he gave the name 

 of head cavities and suggested that they are homologous with the 

 mesoblastic somites of the trunk. The subject was further advanced 

 by Marshall, '78 and '81, and by Van Wijhe, '82, whose paper has 

 been accepted as the standard reference for cephalic segments in the 

 Elasmobranchs. He described nine cephalic mesomeres in Scyllmm 

 and Pristiurus. These observations inaugurated a new era in the 

 investigation of the problem of head segmentation, based upon the 

 study of Selachian embryos. 



