448 EMBEYOLOGY OF THE LOWES VEETEBEATES ch. 



by the same nerve ; and that connected with the ordinary branchial 

 arches there are myotomic muscles as well as splanchnic, so that the 

 basis already exists for a muscularization purely myotomic. 



On the other hand the objection is urged against the Gegen- 

 baur hypothesis that it involves a very great shifting of the 

 pelvic fin backwards from its assumedly original position at the 

 hinder end of the branchial region. This objection need not be 

 taken seriously in view of the extensive shiftings of the limbs which 

 are definitely known to have taken place. Thus in Eays we 

 commonly find that the pectoral girdle-has moved back to a position 

 in relation to the segmentation of the body far posterior to the 

 position which it occupies in Sharks : in Urodele Amphibians the 

 hind-limb has taken up positions, as indicated by the position of the 

 sacrum, varying between the 14th {Triton palmatus) and 63rd 

 vertebra {Amphiuma means) while in the Anura — where in accord- 

 ance with the leaping habits it is advantageous to have the attach- 

 ment of the hind-limb far forward — the sacrum has come to be as 

 far forward as the 9th or even {Hymenochirus) the 6th vertebra 1 : in 

 Plesiosaurs and Birds a still more striking backward migration of 

 the pectoral girdle with its attached limb has taken place {e.g. in 

 the Swan as compared with Archaeopteryx through 14 or 15 

 segments) : and finally in many Teleostean fishes the pelvic fins 

 have become so shifted forwards along the sides of the body as 

 to attain to an actually jugular position. 



The fact that the limb girdles are embedded in the somatopleure 

 while the branchial arches lie in the splanchnopleure has again been 

 raised as a difficulty in the way of accepting the Gegenbaur theory. 

 The difficulty is not so serious as it seems at first sight. The chief 

 obstacle in the way of a splanchnopleural organ becoming shifted 

 outwards into the somatopleure is clearly the coelomic cavity — but 

 in the branchial region this tends to be in great part obliterated. 

 As regards blood-vessels, nerves, etc. — these form by no means 

 insuperable barriers to the change in position of skeletal elements. 

 Such skeletal tissue may, as has already been indicated in Chapter 

 V., spread past a blood-vessel or nerve and if it then becomes 

 absorbed behind the obstacle there is brought about a complete, 

 transposition of the two structures. 



The criticism that the musculature of the limbs is myotomic in 

 origin while that of the branchial arches is splanchnic is provided 

 against by the mixed character of the muscularization of the branchial 

 arches, taken in conjunction with the demonstration that in such a case 

 replacement of splanchnic muscle by myotomic may take place (p. 217). 



Eabl considers the metameric origin of the muscles etc. of the 

 limb to be enough by itself to undermine the Gegenbaur hypothesis, 

 but it is difficult to see how the musculature could be otherwise 

 than metameric in origin seeing that it has to be derived from the 

 segmentally arranged myotomes. 



1 Gadow, in Cambridge Natural History. 



