Johnston, Forebram Vesicle m Vertebrates. 467 



Upon the question of the value of the pre-otic "head cavities" 

 as dorsal somites, ISTeal's statements seem to me very careful and con- 

 servative. With regard to the differentiation of sclerotome and 

 myotome, the preparations seem to me to warrant a positive state- 

 ment that a sclerotome is clearly differentiated in the second or 

 mandibular somite as well as in the third. I am in perfect agree- 

 ment with jSTeal's interpretation of the preotic head cavities, including 

 the anterior cavity of Piatt as dorsal somites. 



Upon the question of a lost branchiomere between the mandibular 

 and hyoid arches, which I have discussed elsewhere (1905) I have 

 been surprised to find so little evidence in the preparations of Squalus. 

 ^^^^lile the third somite lies over the hyomandibular cleft and is 

 somewhat constricted or dumb-bell shaped, its two parts enclose a 

 single cavity and the somite extends over the hyoid arch with the 

 mesodermic core of which it is connected. The fourth somite is 

 smaller than the third and shows that it is rudimentary. AVhile it 

 reaches forward to the hyoid arch, it is connected with the mesoderm 

 of the first branchial arch. The fifth somite has practically corre- 

 sponding relations to the second branchial cleft and the following 

 (second) branchial arch. The sixth somite is clearly connected with 

 the third branchial arch and the seventh somite with the fourth 

 branchial areli. These relations, indeed, may be clearly inferred 

 from N'eal's admirable drawings from cleared specimens. Figs. 15. 

 16, 17. These correspond to the stages from which I have drawn the 

 conclusions stated above. The posterior ends of somites (2), 3, 4. 

 and 5 are connected with their respective branchial arches. 



I must here make reference to a series of papers on the head prob- 

 lem coming from Prof. IT. E. Ziegler's laboratory in Jena (Klink- 

 hardt 1905, Guthke 1906, Ziegler 1908, Brohmer 1909). These 

 authors discuss the fundamental questions of segmentation and the 

 relationship of the cranial nerves. The total material on which the 

 descriptions and conclusions are based consists of ten selachian em- 

 bryos including four of Torpedo between the stage I-K and 20 mm. ; 

 four of Spinax in the stages K, L, M-K, and 7.78 mm. ; one of Chlamy- 

 doselachus in the stage L-]\[ ; and one of Acanthias 22 mm. in length. 



