576 ADAM SEDGWIOK. 
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, Raja, 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, that 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 
divergent observations of Wyhe, 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 the primitive metameric 
segmentation—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 
1 I leave out of consideration the supposed somite anterior to the preman- 
dibular somite (first of Wyhe), which has been described by some observers 
in Acanthias, Torpedo, &c. Ihave seen traces of it in Scyllium, but it 
is in that genus merely a diverticulum of the premandibular somite. 
