ON THE DIPLOCHORDA. 359 



In Balanoglossus there are very similar structures (fig. 23), 

 but the chordoid structure is no longer formed in the two 

 lateral diverticula of the respiratory portion, nor in the 

 pharyngeal clefts, which have now acquired the function of 

 gill-slits. In the former the chordoid tissue is only formed in 

 the anteriorly situated coalesced part, and in the latter case 

 the chordoid walls have been superseded or forestalled by the 

 cuticular branchial skeleton. 



Fig. 24 is a diagrammatic transverse section of the pharynx 

 of a vertebrate, in which the development of the notochord is 

 assumed to resume its primitive order of appearance after 

 the mesoderm has been developed from the endoderm and has 

 taken up its proper position. The two rudiments of the diplo- 

 chordate condition have fused into one median dorsal notochord, 

 still, however, in continuity with the gut. Below this on either 

 side are the widely open gill-slits, and in the mid-ventral line 

 the ciliated groove, destined to be later reduced into the thyroid 

 gland (Gegenbaur). 



In fig. 25 the adult condition of the notochord is reached, in 

 which it has (Eu-chorda) separated from the gut, and be- 

 comes an organ entirely distinct therefrom in form and func- 

 tion. In doing so it pushes dorsalwards into the heemocoele 

 space, and the aorta is thus formed beneath it. (A portion of 

 the pharynx [stomodseum ?] of Cephalodiscus in similar 

 manner protrudes dorsalwards, to come in contact with the 

 ectoderm of the dorsal surface, and in doing so divides the 

 dorsal blood-vessel into two lateral vessels [cf. PI. 24, fig. 121. 

 An extension of this process would lead them to fuse in the 

 mid-ventral line. See Vascular system.) A further stage could 

 be instanced from the higher Vertebrata in which the thyroid 

 also, under the changed conditions of alimentation in the 

 Gnathostomata, loses its connection with the gut-wall. 



We may thus trace the stages from the archichordate pharynx 

 with its walls supported by chordoid tissue, with pharyngeal 

 clefts for escape of the water-current, and with a ventral 

 alimentary portion for the conduction of food particles to the 

 highest Eu-chordate pharynx with the pharyngeal clefts formed 



