398 CHARLES HILL, 



In 1898 the same author reports 9 cephalic segments in Pristi- 

 urus and 10 in Äcanthias. He thus confirms the observations of 

 Van Wuhe and Hoffmann. In Torpedo^ however, he finds 13 meso- 

 dermic segments. According to Sewertzoff one segment in Äcanthias 

 and four in Torpedo have been added to the cephalic region, caudad 

 at the expense of the trunk proper. In this short communication 

 the author refrains from a discussion as to the morphological vakie 

 of the pre-otic segments. 



The review of literature, with exception of the last author quoted, 

 has dealt largely with cephalic metamerism of Selachians. In other 

 Vertebrates, the mesodermic head cavities appear indistinct, and but 

 few observations are recorded upon them. Scott and Osborn, '79, 

 were probably the first to observe series of cœlomic cavities in the 

 head of Amphibian embryos. Miss Platt, '94, finds in Necturus 

 4 pre-otic and 5 meta-otic segments which she considers homologous 

 with the nine conventional segments described by Van Wuhe. Hous- 

 SAY, '90, has observed 11 in the Axolotl, while Oppel, '90, in Anguis 

 embryos describes Van Wijhe's 1st to 3rd and 6th to 9th segments. 

 Lastly, Goronowitsch, '92, finds one pre-otic and two post-otic seg- 

 ments in embryos of the chick. They are very transient and are 

 present in embryos with 6 to 9 somites. 



The review of literature dealing with the cephalic mesomeres 

 makes two points clear: 



1) Observers disagree as to the number of pre-otic segments. 



2) They disagree as to their morphological value. 



The discrepancies in observation and disagreement in inter- 

 pretation of head cavities is more extensive than is usually recognized. 

 The natural presumption is in favor of making the mesomeres of the 

 head carry the marks of primitive segmentation — as they do in the 

 trunk, but it is worth while to keep in mind Sedgwick's suggestion 

 that evidences of primitive segmentation will appear early in develop- 

 mental history and that the metameric arrangement of head cavities 

 as well as nerves and branchial arches may have arisen "subsequently 

 to the disappearance of primitive segmentation". 



b) Segments of the Encephalon. 



A revival of interest in the problem of head segmentation came 



with the relatively recent discovery that the entire neural tube of 



young Vertebrate embryos is divided by transverse constrictions into 



similar segments. The case presented by those segments has been 



