NEAL: nervous system IX SQUALUS ACANTHIAS. 269 



viz. : (1) that the head primitively euded with van "^^'ijhe's 6th somite ; ^ 

 (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 palingenetic 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. 

 Filrbringer). The evidence which I have given seems thus to favor 

 the opinion of Sewertzofif ('95), that we have "keinen Gruud, vorauszu- 

 setzen, dass zwischen den palingenetischen Somiten v. Wijhe's (1-VI) 

 und den coenogenetischen (VII-IX) ein Wegfallen der Segmente statt- 

 gefunden hat. Wir sehen sine vollkommen regelmassige Anlage der 

 Kopfsomiten und ein eben so regelmassiges [ontogenetic] Verschwinden 

 derselben." 



I am aware, however, that the structural differences between the hind- 

 brain neuromeres, e. g. IV to VII, and the neuromeres immediately 



^ 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 Ampliioxus (Hatschek, '92), and by Myxinoids (Price, '96). In this 

 connection, moreover, it is of interest that in my previously ('97) made homology 

 the last visceral cleft in Ammocoetes primitively bounds posteriorly the segment 

 which is homologous with the last cranial segment (HoflTniann, '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 Cyclostomata, differing from that made by me, viz. those by Ahlborn 

 ('84*), by Hatschek ('92), and by Sewertzofif ('95). 



