24 Scientific Proceedings, Royal Dublin Society. 



or attaching themselves to other animals) were acquired. The 

 displacement of the branchial lamellae to the interior explains also 

 the position of the branchial artery internally, instead of externally, 

 to the cartilaginous arch. 



With regard to the comparison which has been drawn by 

 previous authors between the external or extra-branchial arches in 

 selachians, and the branchial arches of cyclostomata, Dohrn has 

 clearly proved that they are not homologous. The branchial arches 

 of the cyclostomata, however, are strictly homologous with the 

 inner or chief arches of selachians, the extra-branchial ones of the 

 latter being merely terminal cartilaginous rays belonging to the 

 inner arches. In describing the cyclostomata, remarks Dohrn, we 

 should, therefore, not follow Gregenbaur and others by saying 

 that they have no jaws because the true inner arches are wanting, 

 but that they possess inner arches, and have lost their jaws, 

 probably on account of the change of the mouth from a biting to 

 a sucking one. 



Gregenbaur argues further that the cyclostomata could not 

 have had limbs, because limbs were transformed gills, and the 

 archipterygium could be reduced to branchial rays. This argu- 

 ment likewise falls to the ground, apart from the invalidity of the 

 archipterygium theory, and we shall presently see that there are 

 still rudiments of the pelvic fin in Petromyzon. 



VI. — The Fins of Elasmobmnchs. 1 



The first stage of the limb formation is recognizable by a skin- 

 fold which, beginning immediately behind the branchial apparatus, 

 reaches as far as the anus. A quantity of mesoderm elements 

 grows into the anterior part of this fold, which at first consists only 

 of ectoderm cells. The same takes place near the anus a little 

 later, and these two projections are the rudimentary pectoral and 

 pelvic fins. 



The mesoblastic somites, or myotomes, lying near the fins, 

 produce two buds each — an anterior and a posterior. These buds, 

 after becoming constricted off from the myotomes, divide again. 

 Hence each myotome produces four separate muscle-masses, which 



1 Ibid., vol. v. 1884, pp. 161-195. 



