270 J. T. CUNNINGHAM. 



the artery ; and the artery is at first posterior to the muscle- 

 tube. The branchial cartilaginous rays arise as condensations 

 of mesoderm cells separate from the arch, and between the 

 artery and posterior vein. The so-called external cartilages 

 are simply the most dorsal and the most ventral of the series 

 of rays altered somewhat in position, and therefore have no 

 similarity with the arches in Petromyzon, which are true arches. 

 Branchial lamellae are never developed on the anterior side 

 of the hyoid arch, or of the spiracular arch. The external 

 filaments of the embryo arise as simple elongations of the 

 posterior lamellae of each arch, the anterior not elongating 

 at all. A curious suggestion is made concerning the function 

 of these elongated filaments, namely, that they serve to absorb 

 yolk ; how the yolk gets into them could not be discovered, 

 but yolk is present in the filaments and in their veins, in the 

 posterior branchial vein, and the eflerent arteries, never in the 

 branchial artery or in the heart. 



Thymus of Elasmobrauchs. 



At the time when the external filaments have attained to 

 about half their length, but when the branchial rays are not 

 differentiated, a proliferation of epithelium takes place in the 

 upper angle of the first gill-cleft, forming a kind of bud. 

 Similar buds are formed in the four posterior gill-clefts, but 

 the fifth bud disappears again entirely in the Sharks, but 

 persists in the Rays. These buds form the thymus of the 

 adult. The cause of the separation of these portions of the 

 branchial epithelium is the shortening of the clefts. The 

 upper portion of the original clefts is obliterated by a coales- 

 cence of the arches, accompanied by processes of growth which 

 alter the original position of the terminal rays of each series, 

 and so produce the extra-branchial cartilages. The epithelial 

 nodules of the thymus after they have sunk into the mesoderm 

 become associated with mesodermic cells, a process which 

 ought not to excite surprise, since the epithelium in question 

 originally no doubt formed branchial laminae into which meso- 

 derm extended. The bending of the arches above described is 



