272 PEOF. G. B. HOWES ON THE DEVELOPMENT OP THE [Mar. 14, 



it are alluded to in Cope's recent ' Batrachia of North America ' ^, 

 except that he refers to the odontoid process of the Urodela 

 (p. 29) as the " body of the pro-atlas ; " Adolphi is silent on the 

 topic, assuming the "sub-occipital" nerve to be absent in all 

 Anura ^ ; while Fiirbringer, in a laborious investigation into the 

 comparative anatomy of the shoulder-muscles, has recorded ' his 

 inability to find a nerve either passing between the skull and the 

 first A'ertebra or perforating the arch of the latter, in any Auuran 

 which he dissected {Pipa excepted). 



Not the least interesting feature in the transverse-process 

 bearing " atlas " of Rana cateshiana (fig. 5 b) and B. esculenta 

 (fig. 4 h) is the presence on the under surface of each of a couple of 

 eminences, weU-nigh indistinguishable from those present in the 

 specimen herein described (fig. 1 a) at the point of fusion of the 

 8th and 9th vertebrae (x). Rana macrodon reveals no such pecu- 

 liarity (fig. 3), but in the Axolotl, in which also I find a " sub- 

 occipital '"' nerve may be present, a deep lyriform depression occurs 

 at the corresponding point (.^% fig. 6), bounded in front by a 

 median tuberosity. While, on exclusively anatomical grounds, these 

 points of similarity support the principle of Hyrtl's conclusion, 

 the facts of comparative morphology ^ that have in recent years 

 shown the hypoglossus nerve-bearing region to be incorporated 

 in the occiput of the Amniota, together with those which are 

 rendering it more and more clear that the os odontoideum of these 

 animals is a true centrum, and the so-called " body" of their atlas 

 an intercentrum ^ forbid further comparison between the " atlas " 

 of the Amphibia and either the atlas or epistropheus of the higher 

 Vertebrata. 



The nerve passing through the arch of the " atlas " in some 

 Urodela conforms, so far as is known, to the characters of a true 

 spinal nerve ; as described by Humphry in Jlegalobatrachus, it has 

 the relations rather of a ventral than a dorsal ramus of a typical 

 nerve of the trunk ; while, as described by Fischer for Menohninchus, 

 the reverse would appear to hold good. Prof. Stewart informs me 

 that in Rami cateshiana (the " atlas " of which is herein figured) 

 he traced it to the muscles of the hyoid region. 



Stohr has recently shown " that the odontoid process of the 

 Amphibian (Triton) is a primarily independent derivative of the 

 cranial notochord ; and he lays much stress '' upon the conclusion 

 that the homologues of the hypoglossus and accessorius nerves 

 of the higher Vertebrata are to be sought in the anterior spinal 

 nerves of the lower ones. If this be accepted, the known 



' Bull. U.S. Nat. Mus. no. 34 (1889). 



- Loc. cit. p. 316. 



^ Jenaische Zeitsebr. Bd. vii. p. 286 (1873), and Bd. viii. p. 180 {cf. v. Ihering, 

 infra, p. 273). 



" For Ari-sume of these see P. Z. S. 1890, p. 358. 



° Cf. especially Baur, Biolog. Centralbl. Bd. vi. p. 359, and Boulenger, Ann. 

 & Mag. Nat. Hist. ser. 6, vol. iii. p. 140. 



'^ Zeitschr. f. wiss. Zool. Bd. sxxvi. p. 99, 



'' Ibid. Bd. xsxiii. p. 518. 



