22 Scientific Proceedings, Royal Dublin Society. 



between the diverticula of the alimentary tract, from which the 

 gill-clefts are formed. Balfour has already pointed out that the 

 original gill-arches in Petromyzon do not lie near the distal end, 

 as in selachians, but are in close proximity to the proximal end or 

 inner border of the visceral arches. The muscle-tube, which is 

 originally round, elongates, causing the component cells to group 

 themselves into two rows, which will be spoken of as the visceral 

 and parietal respectively. At the same time the cartilaginous 

 arch is differentiated from the mesoderm tissue on that side, lying 

 externally to the parietal row just mentioned. The visceral wall 

 of the muscle-tube now becomes thickened more strongly than the 

 parietal, and a proximal portion separates off from a distal part 

 of the mesoblastic somite, the cartilaginous arch appearing between 

 the two. 



While the arch undergoes a series of curvatures in various 

 directions, a histological differentiation of a peculiar kind is noticed 

 in the two muscular layers. The visceral muscle shows a most 

 perfect striation, while the cells of the parietal become drawn out 

 into long tubes, persisting in their embryonic form and exhibiting 

 only an external striation. 



All muscles belonging to the visceral arches spread along the 

 whole circumference of the branchial cavity. They are inserted 

 dorsally to the chorda dorsalis ; but, both dorsally and ventrally, 

 the arches of each side unite with one another. The muscles are 

 inserted comparatively late in various regions of the cartilaginous 

 rods. At first they pass by the rods and unite dorsally and 

 ventrally with one another. This is of great importance, as will 

 be shown later on, when Myxine comes to be considered. In the 

 latter, indeed, where the rods have disappeared, this condition of 

 the muscles still remains. The great difference of the branchial 

 apparatus of elasmobranchs, and that of Petromyzon, consists in 

 the gill-lamellae of the former being directed outwards, and inwards 

 in the latter. 



The external clefts are probably opened by the action of the 

 adductor muscles. On the other hand, the long muscles of the 

 distal parts — the constrictor muscles — are perhaps expiratory in 

 function. 



We thus find that the gill-arches in Petromyzon are completely 

 homologous with the inner arches of elasmobranchs. This is con- 



