THYMUS-LIKE STRUCTURES IN AMMOCOETES 135 



It differs from the conditions in amphioxus in that the gill 

 clefts are very much wider. It is probable that the ammocoete 

 gill cleft represents the fusion of two or more gill clefts of am- 

 phioxus. There is an indication of such a fusion in the for- 

 mation of the tongue-bar or secondary gill bar of amphioxus. 

 The larger respiratory part of the ammocoete pharynx (text 

 fig. 1, r.p.) corresponds to the peribranchial cavity of ascidians 

 and amphioxus. The entire branchial cavity of the petromyzon 

 larva is entodermal in origin. The ammocoete thus represents 

 a phylogenetic stage in which the respiratory cavity, originaliy 

 of ectodermal origin, is derived from the endoderm as it is in 

 most higher animals. 



The elasmobranch pharynx, it seems to me, represents an 

 advanced stage of a modification which is already indicated in 

 the ammocoete. This modification consists of a lateral migra- 

 tion of the dorsal and ventral attachments of the gill arches, 

 resulting in a lateral enlargement of the central portion of the 

 pharyngeal cavity and a consequent reduction of the respiratory 

 part. This lateral migration is indicated by the dorsal at- 

 tachments of the second pair of gill arches in the lamprey larva. 



Attention may also be called to the fact that the primitive 

 elongated character of the pharynx in the ammocoete tends to 

 obscure its relation to the elasmobranch pharynx, in which the 

 length has been reduced with a consequent reduction of the 

 number and size of the gill slits. 



It is necessary to determine the character of the gills in ammo- 

 coetes in so far that it has been established that the thymus in 

 all higher forms has a more or less definite relationship to the 

 gill pouches and gill arches. 



Text fig. 1 Model of a segment of the branchial region of a 15 mm. lamprey 

 larva. Cephalic aspect. The model shows the relationship of the primitive 

 thymus placodes to the epipharyngeal ridge and the ciliated epithelium, as well 

 as the relationship of the epipharyngeal ridge to the gill arches and general 

 branchial cavity, a., atropore; a.p., alimentary pharynx; c.c, ciliated epithe- 

 lium; d.a., dorsal aorta; ep.r., epipharyngeal ridge; g.a., gill arch; g.L, gill lamel- 

 lae; n., notochord; p.t.p., primitive thymus placode; /•./;., respiratory pharynx; 

 S.C., spinal cord; v. a., ventral aorta; .r., position of accumulations of Ij-mphocytes 

 in lateral branchial walls. 



