NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 269 
viz. : (1) that the head primitively ended with van Wijhe’s 6th somite ; 1 
(2) that between this and the following somite segments (dorsal as 
well as ventral) have been phylogenetically lost ; and (3) that the head 
primitively ended with the gill region. It has been shown, I believe, 
that the probable phylogenetic and actual ontogenetic disappearance of 
visceral clefts does not necessitate the loss of the corresponding mesomere 
and neuromere. It is true that we have very good reason to infer a 
phylogenetic loss of distinctly differentiated somites and neuromeres in 
the Vertebrate series. It is also true that we find evidence of an onto- 
genetic disappearance of mesomeres. Nevertheless such evidence does 
not prove that somites have been phylogenetically lost from the occipital 
region before the group of Selachii, of which Squalus is one of the most 
primitive forms, is reached. I believe that the evidence which has 
been given of the complete metameric correspondence of neuromeres and 
mesomeres — that the Selachian embryo is in this respect an unbroken 
continuum — renders it unnecessary to assume that somites have been 
so completely lost that no traces of them appear phylogenetically in 
Selachii. It is no longer necessary to assume a palingenctic portion 
of the Vertebrate head which ended with the sixth visceral arch of 
Selachii (Gegenbaur), or an exact homology between the hypoglossus 
roots (surely a most uncertain “fixed point”) of adult Vertebrates (M. 
Fürbringer). The evidence which I have given seems thus to favor 
the opinion of Sewertzoff (95), that we have “keinen Grund, vorauszu- 
setzen, dass zwischen den palingenetischen Somiten v. Wijhe’s (I-VI) 
und den coenogenetischen (VII-IX) ein Wegfallen der Segmente statt- 
gefunden hat. Wir schen sine vollkommen regelmässige Anlage der 
Kopfsomiten und ein eben so regelmässiges [ontogenetic] Verschwinden 
derselben.” 
Tam aware, however, that the structural differences between the hind- 
brain neuromeres, e. g. IV to VII, and the neuromeres immediately 
1 The suggestion that the gill region is not confined to the head region was first 
made by Huxley (’58). I believe that direct evidence in favor of this suggestion is 
furnished by Amphioxus (Hatschek, ’92), and by Myxincids (Price, ’96). In this 
connection, moreover, it is of interest that in my previously (’97‘ made homology 
the last visceral cleft in Ammocotes primitively bounds posteriorly the segment 
which is homologous with the /ast cranial segment (Hoffmann, ’94) of Squalus, viz. 
van Wijhe’s 10th somite. Furthermore it has been shown (p. 268) that this last 
visceral cleft of Petromyzon is exactly homologous with the last visceral cleft of 
Amphioxus in its “critical stage” of development. It should, however, be noted 
that there have been published three other interpretations of homologies between 
Selachii and Cyclostometa, differing from that made by me, viz. those by Ahlborn 
(784), by Hatschek (’92), and by Sewertzoff (’95). 
