ALIMENTARY CANAL. 



759 



In addition to internal gills external branchial processes covered 

 by epiblast are placed on certain of the visceral arches in the 

 larva of Polypterus, Protopterus and many Amphibia. The 

 external gills have probably no genetic connection with the 

 internal gills. 



The so-called external gills of the embryos of Elasmobranchii 

 are merely internal gills prolonged outwards through the gill 

 clefts. 



The posterior part of the primitive respiratory division of the 

 mesenteron becomes, in all the higher Vertebrata, the oesophagus 

 and stomach. With reference to the development of these parts 

 the only point worth especially noting is the fact that in 

 Elasmobranchii and Teleostei their lumen, though present in 

 very young embryos, becomes at a later stage completely filled 

 up, and thus the alimentary tract in the regions of the 

 oesophagus and stomach becomes a solid cord of cells (fig. 23 

 A, ces}\ as already suggested (p. 61) it seems not impossible that 

 this feature may be connected with the fact that the cesophageal 

 region of the throat was at one time perforated by gill clefts. 



In addition to the gills two important organs, viz. the 

 thyroid body and the lungs, take their origin from the respi- 

 ratory region of the alimentary tract. 



Thyroid body. In the Ascidians the origin of a groove- 

 like diverticulum of the ventral wall of the branchial sack, 

 bounded by two lateral folds, and known as the endostyle or 

 hypopharyngeal groove, has already been described (p. 18). 

 This groove remains permanently open to the pharyngeal sack, 



a 



FIG. 414. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA 



OF PETROMYZON. (From Gegenbaur ; after Calberla.) 



o. mouth ; 6 '. olfactory pit ; v. septum between stomodanun and mesenteron ; 

 h. thyroid involution ; n. spinal cord ; ch. notochord; c. heart ; a. auditory vesicle. 



