760 



THE THYROID BODY. 



and would seem to serve as a glandular organ secreting mucus. 

 As was first pointed out by W. Mtiller there is present in 

 Amphioxus a very similar and probably homologous organ, 

 known as the hypopharyngeal groove. 



In the higher Vertebrata this organ never retains its primi- 

 tive condition in the adult state. In the larva of Petromyzon 

 there is, however, present a ventral groove-like diverticulum of 

 the throat, extending from about the second to the fourth 

 visceral cleft. This organ is shewn in longitudinal section in 

 fig. 414, h, and in transverse section in fig. 415, and has been 

 identified by W. Muller (Nos. 565 and 566) with the hypo- 

 pharyngeal groove of Amphi- 

 oxus and Ascidians. It does 

 not, however, long retain its 

 primitive condition, but its open- 

 ing becomes gradually reduced 

 to a pore, placed between the 

 third and fourth of the perma- 

 nent clefts (fig. 416, th). This 

 opening is retained throughout 

 the Ammoccete condition, but 

 the organ becomes highly com- 

 plicated, with paired anterior 

 and posterior horns and a 

 median spiral portion. In the adult the connection with the 

 pharynx is obliterated, and the organ is partly absorbed and 

 partly divided up into a series of glandular follicles, and event- 

 ually forms the tJiyroid body. 



From the consideration of the above facts W. Muller was led 

 to the conclusion that the thyroid body of tlie Craniata was 

 derived from the endostyle or hypopharyngeal groove. In all the 

 higher Vertebrata the thyroid body arises as a diverticulum of 

 the ventral wall of the throat in the region either of the mandi- 

 bular or hyoid arches (fig. 417, Th), which after being segmented 

 off becomes divided up into follicles. 



In Elasmobranch embryos it appears fairly early as a diverticulum from 

 the ventral surface of the throat in the region of the mandibular arch, 

 extending from the border of the mouth to the point where the ventral aorta 

 divides into the two aortic branches of the mandibular arch (fig. 417, Th). 



FIG. 415. DIAGRAMMATIC TRANS- 

 VERSE SECTIONS THROUGH THE BRAN- 

 CHIAL REGION OF YOUNG LARWE OF 



PETROMYZON. (From Gegenbaur ; after 

 Calberla.) 



d. branchial region of throat. 



