OF THK UEAU IX THE BATKACHIA URUDKLA. 117 



segments, not the homologue of a single neural arch a,s Stohr 

 supposed. Fiirbringer adopts much the same view in his great 

 monogi'aph (2), concluding that the occipital condyle of the 

 Amphibia occupies the same segmental position as the last 

 occipital segment in the Selachii. Many segments are supposed 

 to have been crushed out between this and the vagus corre- 

 sponding to the region in the Selachian skull through which 

 pass the spino-occipital nerves. He claims, indeed, to have found 

 possible ti'aces of these segments in C'rypioh-anchus, where he 

 discovered a smalh ventral nerve-root {z.) piercing the occipital 

 bone. Miss Piatt and Gaupp (8 and 3) are inclined to accept 

 Fiirbringer's view. IS'ow the adoption by so many anatomists of 

 the theory that a number of segments have vanished from the 

 Amphibian head by a pi-ocess of excalation is, I venture to think, 

 based not so much on facts as on theoretical considerations. 

 In the first place, there is a reluctance to admit that a structure 

 like the occipital condyle can be homologous in two groups, 

 although developed in different segments. But there can be no 

 doubt that the homology of an organ is independent of its 

 position in the segmental series. 



The hind limbs of a Frog, an Axolotl, and an Amphiuma are 

 homologous, in spite of the fact that they are placed in different 

 segments. It is unnecessary here to recapitulate in full argu- 

 ments which have ali'eady been given at length elsewhere (5, 6) 

 with regard to the development of the fins of fishes. But I may 

 briefly state : — That every trunk-segment is capable of producing 

 limb-elements ; that is to saA', of contributing to the formation of 

 median and paired fins. The shifting of a fin up or down the 

 body is not due to the migration of fin -material from one place to 

 another in the course of ontogeny ; the fin, as a whole, arises 

 from that region of the trunk which it occupies in the adult (as 

 is shown by the development and the nerve-supply). Relative 

 displacement in ontogeny is due almost entirely to " concentra- 

 tion," a relative narrowing of the base of the fin. Change of 

 position in phylogeny is brought about by progressive reduction 

 on one side, and growth on the other ; apparent migration is due 

 to certain segments beginning to contribute to the fin at one 

 end and certain other segments ceasing to contribute at the 

 other. By such " transposition " the fin may reach an entirely 

 new position. 



Kow, in the case of the fins of fishes, I have already shown (5) 

 that it is not possible to account for variation in position by 

 the theoiy of inter- and excalation. Growth and transposition 

 from one segment to another alone account for the facts. The 

 same is probably true of the occipital condyle. Thei'e is not the 

 slightest trace of the disappearance of segments behind the vagus 

 in the ontogeny of the Amphibia. We are familiar with tiie 

 variation in the extent of the gill-region in Yertebratesby mei'e 

 growth. Obviously tlie hind limit of the series of gill-slits varies 

 1)ack\\"ard . or forward, according •. as cLTtain.: segmcuts. cease'.tu 



