396 CHARLES HILL, 



In the pre- otic region of Torpedo marmorata, 3 mm long, Dohrn, 

 '90, finds 13 to 15 segments, which, taken with the four post-otic 

 segments , that he considers homologous with Van Wijhe's 6—9 

 somites, makes a total of 18 to 19 cephalic mesoraeres. Killian, '91, 

 confirms this with one correction finding 17 to 18 segments. Miss 

 Platt, '91, adds an anterior cavity in Acantkias vulgaris to Van 

 Wijhe's nine mesomeres. She also finds one cephalic segment caudad 

 and reports one between Van Wijhe's 3rd and 4th segments making 

 a total of 12 cephalic mesomeres. HofFxMann, '94, reports ten in 

 Acanthias adding one caudad to the 9th observed by Van Wijhe. 



Van Wijhe, Dohrn, Killian, Miss Platt and Hoffmann, regard 

 these cephalic segments as serially homologous with the somites of 

 the trunk proper. 



Sedgwick, '92, records a variable number of cephalic mesomeres 

 and doubts their segmental value. In support of this view he offers 

 the following argument (in: Quart. Journ. raicr. Sc, V. 33, p. 576): 

 "But even if Dohrn is right in his enumeration of the anterior somites, 

 it is clear that Torpedo differs from Scyllium, Raja and Pristiurtis, 

 whether my account or Van Wijhe's be taken as correct. For in 

 Torpedo there are four somites where in the other genera there is 

 most unquestionably one, e. g. the somite of Wijhe. It would appear, 

 then, that if the number of primitive cranial somites in any given 

 region of the head does really difier in closely allied genera in the 

 manner indicated by the divergent observations of Van Wijhe, Dohrn, 

 Killian and myself, the supposed indications of segmentation which 

 are found in the adult and are constant throughout the Vertebrata 

 can have very little value as real tests of primitive metameric seg- 

 mentation. We may, I think, even go further and say that the adult 

 arrangement of nerves and branchial arches, etc., characteristic of the 

 Vertebrate head, must have arisen subsequently to the disappearance 

 of primitive segmentation." 



Kupffer, '92, who has given much study to the head cavities in 

 Acipenser and Ammocœtes writes as follows (in : Ergebn. Anat. Entw., 

 V. 2, p. 522) : "I assume that these head-cavities represent rudimentary 

 gill pockets which upon the disappearance of the old mouth or Pa- 

 lœostoma, lost their function. On the contrary their origin as direct 

 diverticles of the fore-gut clearly indicates that they are visceral clefts." 



It is a significant fact that Hatschek, '92 (according to Kupffer, 

 '92), has found seven pair of entodermic evaginations in the cephalic 

 region of Amphioxus, which in later stages become constricted from 



