vii DEVELOPMENT OF LIMBS 443 



whether any record of it occurs in ontogeny. Obvious evidence 

 which at once suggests itself in this connexion is the presence of 

 abortive muscle-buds in front of or behind those which become 

 incorporated in the definitive limb. These seem clearly to in- 

 dicate that the part of the body surface superficial to these buds 

 was at one time part of the actual limb. But unfortunately such 

 abortive buds occur both anterior and posterior to the definitive 

 limb and there is no means of fixing definitely the time in phylo- 

 genetic evolution from which the two sets of abortive buds date. 

 They may date from the same period, in which case they might 

 merely afford evidence of the process of narrowing of the limb base 

 which has undoubtedly taken place during the later evolution of 

 fins ; or they may date from different periods, in which case the 

 anterior set might be taken as evidence of a backward movement of 

 the limb, and the hinder set as evidence of a forward movement 

 occurring at a different period — movements which again have un- 

 doubtedly taken place. In view of the impossibility of determining 

 to what extent the evidence in any particular case is to be inter- 

 preted in these two different directions it seems on the whole advis- 

 able to leave this muscle-bud evidence on one side. 



Other evidence has been adduced from cases where the actual 

 limb rudiment as a whole (i.e. the projection from the surface of the 

 body) seems to be displaced during development. For example in 

 the figures illustrating the development of the pelvic limb in Spinax 

 (p. 207) it will be seen that the anterior limit of the fin is in successive 

 stages of development opposite myotomes 21, 26, 28 and 31 ; the 

 hinder limit at the same stages opposite myotomes 30, 38, 38 and 

 39, and the middle of the fin base opposite myotomes 25, 32/33, 

 33/34 and 35/36. It seems quite allowable in such a case to speak 

 of the limb as having undergone a backward displacement. In other 

 cases, as that of Scyllium according to Goodrich (1906), ontogenetic 

 development discloses no evidence of such backward migration. 



The limb rudiment gradually increases in size and assumes 

 its definitive form and as it does so it becomes equipped with its 

 characteristic skeletal and neuromuscular arrangements in the manner 

 already described. 



Phylogenetic Origin of the Limbs. — The limbs are organs 

 highly characteristic of the Vertebrata. While they exist typically 

 as two pairs, pectoral and pelvic, one or both pairs readily disappear 

 in groups where they are no longer needed. They are particularly 

 prone to disappear in those Vertebrates which assume an elongated 

 form of body and revert to the archaic method of moving by lateral 

 flexure. Thus in Eels the pelvic limb has disappeared, in Symbranchus 

 both pairs. In Lepidosiren, the most elongated Lung-fish, we see 

 both pairs in process of reduction. We see the same in elongated 

 Urodeles such as Amphiuma, while in the Gymnophiona both pairs 

 have vanished. In Reptiles we see beautifully how the limbs undergo 

 reduction (Chalcides) and complete disappearance (Amphisbaenidae, 



