142 MERISTIC VARIATION. [part I. 



second pair leaves between the second and third vertebrae, and the 

 third pair leaves between the third and fourth vertebrae. The brachial 

 plexus is formed by the whole of the second pair together with parts of 

 the first and third pairs. (The details of the arrangement are compli- 

 cated and vary greatly in different forms.) In Pipa a different arrange- 

 ment exists. The most anterior pair of nerves leaves the spinal 

 column by perforating the first vertebra, and the pair which leaves 

 between the first and second vertebra? is therefore ordinally the second 

 pair of spinal nerves in this form; the pair which leaves between the 

 second and third vertebrae is the third, and so on. The brachial plexus 

 is made up of the whole of the second nerve, nearly the whole of the 

 third nerve and of a branch of the first. 



If then it were to be supposed that the pair of nerves which leaves 

 the column between the first and second vertebrae in Pipa is homo- 

 logous with the pair of nerves which leaves in the same place in Rana, 

 ifcc, it is clear that between the skull and the 2nd vertebra of Pipa, 

 there is an extra pair of nerves not found in Rana. The number of 

 free vertebrae in Pipa is however less than in Rana. For in the former 

 there are only seven of these, making with the united sacral vertebra 

 and urostyle eight pieces in all ; but in Rana there are eight pre- 

 sacrals, one sacral, and counting the urostyle, ten pieces in all. In 

 Rana only one spinal nerve, the 10th, leaves the urostyle, while in 

 Pipa two pairs, the 9th and 10th, pass out through the terminal piece 

 of the vertebral column, suggesting that the diminution in the 

 number of vertebrae is due to the absence of separation between the 

 9th vertebra and the urostyle. The whole number of spinal nerves 

 is therefore- the same in both Rana and Pipa, but in the latter the 

 1st pair perforate the 1st vertebra in addition to the 2nd pair 

 which pass out between the 1st and 2nd vertebrae. Furbringer 1 , 

 M., Jen. Zt., 1874, vm. p. 181 and Note, PL vn. fig. 37 ; also Jen. 

 Zt., 1873, vii. PI. xiv. figs. 5 and 6. 



It was suggested by Stannius (Lehrb. d. vergl. Anat., p. 130, Note) 

 that perhaps the 1st vertebra of Pipa represents two coalesced verte- 

 brae, but in an anatomical examination of two specimens of Pipa, 

 Furbringer (I.e. 1874, p. 180), found no confirmation of this suggestion, 

 and developmental evidence also went to shew that no such fusion 

 occurs in the ontogeny at least 2 . Kolliker, A., Verh. phys.-med. 

 Ges. Wilrzburg, 1860, x. p. 236. 



As Furbringer says there is no satisfactory way of bringing this case 

 of Pipa into accord with the condition seen in Rana. In the Urodela 

 there is of course a suboccipital nerve between the skull and the 1st 

 vertebra which is not present in Rana, and some resemblance to Pipa 

 is thus suggested; but in the Urodela the 1st spinal does not actually 



1 Compare von Jhering, H., Morph. Jahrb., 1880, vi. p. 297. The statement 

 made by von Jhering that the nerves of Pipa and Rana correspond nerve for nerve, 

 though in different positions relative to the vertebras, if established would be 

 important ; but from the want of detailed description it is not clear whether this 

 conclusion was arrived at by actual dissection. 



2 This is questioned by Adolphi, Morph. Jahrb., xix. 1892, p. 315, note. The 

 same paper contains much important matter bearing on the variation of the nerves 

 of Amphibia. I regret that this paper did not appear in time to enable me to 

 incorporate the facts it contains. 



