352 PROCEEDINGS OF THE ACADEMY OF [1887. 



with great difficulty that the white and gray matter can be distin- 

 guished in sections, or that any differentiation, except around the 

 central canal, is visible. 



In the singular and remarkable case of the Cetacea the emliryo- 

 logical evidence, here offered, is entirely reconcilable with the views 

 for the first time propounded by the writer upon morphological 

 grounds, that the distal portions of the hind limbs are represented 

 by the flukes. The latter being in reality the outward vestiges of 

 hind limbs, so that the statement in recent text-books to the eflect 

 that, "the Cetacea are without hind limbs," must be qualified. The 

 morphological evidence attainable proves beyond a resonable doubt 

 that the distal part of the hind limbs have been translocated back- 

 wards into their present position in Cetaceans, through the inter- 

 mediation of a type approximating the existing pinnipeds, in which 

 a similar process is now taking place. 



In the Cetacea, the translocation of the hind limbs has been in a 

 backward direction or just the reverse of what has occurred in the 

 physoclistous Fishes. The Cetacean "lumbo-caudal plexus" which 

 at least furnishes the sensory branches of nerves going to those or- 

 gans, is therefore, either a backwardly ti-anslocated structure, simi- 

 lar in character to the forwardly translocated pairs of nerves going 

 to the pelvic limbs of Physoclists, or it may be that they represent 

 the modified posterior part of tlie system of spinal nerves, which 

 supply the muscles of the powerful tail and have thus acquired 

 secondarily a more intimate relation to the flukes. At any rate, the 

 nerves, in this case, give us a far less potent argument in favor of 

 translocation than do the skeleton and muscles, which are alone con- 

 clusive, when contrasts are made between their condition in the 

 normal Mammalia, the pinnipeds and the Cetacea as the last extreme 

 of modification. 



But backward translocation of limbs is not confined to Cetacea. 

 In all fishes so far observed by competent embryologists the pec- 

 torals grow out on either side of the anterior end of the trunk as a 

 pair of folds just behind the last pair of branchial arches. In one 

 group however, the embryology of which is not known, and which 

 will in all probability remain inaccessible to us for the reason that, 

 both Gastrostomus and Ophiognathus, the genera referred to, are 

 abyssal forms, there is every reason to believe that the pectoral 

 pair of fins has been translocated backwards. In the case of Gas- 

 trostomus bairdii, this translocation has pushed the pectoral fins 



