448 EMBEYOLOGY OF THE LOWEE VEETEBEATES OH. 



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 musculari/ation 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 lie 

 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 (Ampliiuma 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 l : 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 myolnmie in 

 origin while that of the branchial arches is splanchnic is provided 

 against by the mixed character of the muscularization of the hranehial 

 arches, taken in r.nnjunet.iim wit h the demonstration that in such a ease 

 replacement of splanchnic muscle h\ myot-omic may lake plae (]>. 1' 1 7). 



11,-ibl considers the iin-lanirrie mi^m of the muscles etc. <>1 the 

 limb to be enough by itx-ll'to undermine Ihe (le^enhaur h\ pot he-is. 

 but it in dillicull to see how tin- musculature could he otherwise 

 than me|:iim-nc in origin seeing thai, it has t< he derived I'nuu the 

 seginenlally m ynlnines. 



1 Gadnw, in (.',nii /'//// 



