SVOiiUTION OF OEGANS IN THE CHORDATA. 271 



connected with the fornaation of the united portions of the 

 musculus constrictor superficialis, but the original cause of the 

 whole process is to be explained only after further investiga- 

 tions have been described. Ecker first definitely described the 

 thymus of fishes in his article " Blood-vessel Glands," in Wag- 

 ner's ' Dictionary of Physiology,' Bd. iv, but could find no such 

 organ in the Sturgeon, in Cyclostomata, or in Teleosteans. 

 In a foot-note Dohrn points out that the thymus of Teleosteans 

 exists in the position already accurately defined by Leydig in 

 his 'Anat. histol. Untersuchungen iiber Fische und Reptilien/ 

 In this note also emphatic contradiction is made of Gegen- 

 baur's generally accepted view that the pseudobranchia of 

 Teleosteans is the reduced gill of the hyoid arch, and therefore 

 not homologous with the pseudobranchia or spiracular gill of 

 Elasmobranchs. Dohrn maintains that Johann Miiller was 

 quite right in asserting that the pseudobranchia of Teleosteans 

 was homologous with the spiracular gill of Elasmobranchs, 

 and that Balfour, who has been followed by Hoffmann, was 

 mistaken in supposing that in the Teleosteans the choroid 

 gland represents the spiracular gill. Stieda found that the 

 thymus of mammals arose from only one gill-cleft, the last, or 

 last but one ; Dohrn states that the carotid gland may possibly 

 represent a rudimentary thymus derived from another cleft. 



Branchial Skeleton and Arches of Petromyzon. 



After showing that the extra-branchial cartilages of Elasmo- 

 branchs are really displaced gill rays, the next point in arguing 

 that the branchial skeleton of Petromyzon is composed of 

 true branchial arches, is to demonstrate the development of 

 this skeleton, and this is the object of the fifth paper. It is 

 known from the researches of Scott and Balfour that the 

 first trace of the visceral arches appears in the form of 

 head-cavities, rounded cell-tubes between the diverticula of 

 the gut, which afterwards form the gill-clefts. The question 

 of correspondence between the head- cavities and the dorsal 

 myotomes is left for a future period. There is a difference 

 between the embryonic gill arches of Petromyzon and those 



