NOTES ON BLASMOBRANCH DEVELOPMENT, 251 



through any one series of sections it is very easy to make out 

 what appears to be a great number of somites, but on care- 

 fully comparing the two sides of the embryo, and on estimating 

 the intervals which the somites occupy, it is in my experience 

 always found (after Stage F in the head region) that, with the 

 exception of the first three head segments and the three poste- 

 rior segments, these supposed somites are in embryos, in which 

 the rudiments of the spiracle and hyoid cleft are apparent, 

 quite irregular, and are either simply spaces in the mesoderm 

 or remains of broken-down somites. This result comes out 

 still more forcibly if one attempts to confirm one's observa- 

 tions on one embryo by similar observations on another embryo 

 of the same size. 



But even if Dohrn is right in his enumeration of the 

 anterior somites, it is clear that Torpedo differs much from 

 Scyllium, Eaja, and Pristiurus, whether my account or 

 Wyhe'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 Wyhe.^ 



It would appear, then, if the number of primitive cranial 

 somites in any given region of the head does really differ 

 in closely allied genera in the manner indicated by the diver- 

 gent observations of Wyhe, Dohrn, Killian, and myself, that 

 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 the primitive metameric seg- 

 mentation — i. e. of the segmentation which obviously persists 

 in the trunk region, and which begins with the segmentation 

 of the mesoderm, and is moulded upon it in the manner cha- 

 racteristic of all metamerically segmented animals. 



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

 arrangements of nerves and branchial arches, &c,, character- 

 istic of the Vertebrate head, must have arisen subsequently to 



' I leave out of consideration tiie supposed somite anterior to the preman- 

 dibular somite (first of Wjlie), wliicli lias been described by some observers 

 inAcanthias, Torpedo, &c. I have seen traces of it in Scyllium, but it 

 is in that genus merely a diverticulum of the premandibular somite. 



