318 EMBRYOLOGY. 



which is the equivalent of the evagination of the pharyngeal epithelium in the 

 remaining Vertebrates. By the further approximation of the ridges the depres- 

 sion becomes an epithelial sac, which remains for a long time in communication 

 with the surface of the tongue by means of a narrow passage, the ductus 

 thyreoglossus. 



The paired fundaments of the thyroid gland were discovered a few 

 years ago by STIEDA in mammalian embryos, but they have been 

 more fully investigated by BORN, His, KASTSCHENKO, DE MEURON, 

 and others in Mammals and other Vertebrates (excepting Cyclo- 

 stomes). In the Amphibia, as well as in Birds and Mammals 

 (fig. 176 S), there are formed, a little while after the appearance of 

 the unpaired fundament, two hollow evaginations of the ventral 

 epithelium of the throat behind the last visceral arch and in con- 

 nection with the last visceral cleft. They come to lie immediately on 

 either side of the entrance to the larynx. In many Reptiles (fig. 176 

 A nsd) there is an interesting deviation due to the fact that an 

 evagination is developed only on the left side of the body, while on 

 the right it has become rudimentary. Even in the Selachians 

 (fig. 175), as DE MEURON appears rightly to maintain, paired 

 fundaments of thyroid glands are present. They are the previously 

 mentioned supra-pericardial bodies discovered by v. BEMMELEN. These 

 arise as evaginations of the epithelium of the throat behind the last 

 pair of gill-clefts near the anterior end of the heart. In all cases 

 the evaginated portions of the epithelium become detached from 

 their parent tissue and enclosed on all sides by connective tissue ; 

 they then undergo a metamorphosis similar to that of the unpaired 

 fundament of the thyroid gland. 



In regard to their ultimate position there exist considerable 

 differences between the separate classes of Vertebrates. In the 

 Selachians the supra-pericardial bodies remain far away from the 

 unpaired thyroid gland, being located in the vicinity of the heart ; 

 but in the other Vertebrates they move more or less close to the 

 gland, and have here acquired the name of accessory thyroid glands 

 (fig. 177 A and B nsd}. Finally, in Mammals and Man the approxi- 

 mation has led to a complete fusion of the unpaired and the lateral, 

 paired fundaments (fig. 177 G). Together they constitute a horse- 

 shoe-shaped body that embraces the larynx. It is, however, to be 

 observed, that at the time of their fusion the lateral fundaments, 

 in comparison with the median one, are only very small nodules. 

 Consequently KASTSCHENKO, who is probably in the right, ascribes to 

 the former an inconsiderable importance for the development of the 



