20 



Scientific Proceedings, Royal Dublin Society. 



one another. The " basale " (pharyngo-branchial) is constricted 

 off from the dorsal, and the "copulare" (hypo-branchial) from 

 the ventral piece. (See fig. vi.) 



The branchial cartilaginous rays are formed considerably later 

 than the arches. They arise as condensations of mesoderm cells 

 between the branchial artery and the posterior vein close to the 

 cartilaginous arch. Later on the rays approach the arch still 

 more, without, however, becoming anchylosed with it. These rays, 

 except the two external ones, are in connexion with the two middle 



Diagrammatic figure of a cross section of two Branchial Arches (Elasmobranch). 



g. s. = ext. gill-slit, 

 g.p. = gill-pouch, 

 p.l. = post, gill-lamella. 

 a. 1. = ant. gill-lamella, 

 p. v. = post. vein. 



a. v. = ant. vein. 



c. a. = cartil. arch. 

 c. r. = cartil. ray. 

 n. = nerve. 



b. a. = branchial art. 



pieces of the arch. While all other rays grow straight out, these 

 two external ones (one dorsal and the other ventral) receive from 

 their very beginning a curvature. As they grow longer they curve 

 still more — in fact they grow towards one another. This extraordi- 

 nary growth is probably due to the gradual diminution in size of 

 the branchial clefts. These dorsal and ventral external rays consti- 

 tute the so-called external cartilages. By all previous observers, 

 including Balfour and G-egenbaur, these cartilages were looked 

 upon as being equivalent with the arches in Petromyzon. Most of 

 their speculations were based upon that fact. Dohrn, however, 



