624 



THE THYROID BODY. 



In Ascidians the respiratory sack is homologous with the respira- 

 tory tract of Amphioxus. 



The details of the development of the branchial clefts in the differ- 

 ent groups of Vertebrata have already been described in the systematic 

 part of this work. 



In all the Ichthyopsida the walls of a certain number of clefts be- 

 come folded ; and in the mesoblast within these folds a rich capillary 

 network, receiving its blood from the branchial arteries, becomes 

 established. These folds constitute the true internal gills. 



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 alimen- 

 tary tract in the regions of the oesophagus and stomach becomes a 

 solid cord of cells (fig. 23 A, oes)-. as already suggested (p. 50) it seems 

 not impossible that this feature may be connected with the fact that 

 the oesophageal 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 respiratory 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. 15). This groove remains permanently 

 open to the pharyngeal sack, and would seem to serve as a glandular 



Fig. 414. Diaghammatic vertical section of a just-hatched larva op 

 Petromyzon. (From Gegenbaur; after Calberla.) 

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

 li. thyroid involution ; w. spinal cord; ch. notochord; c. heart; a. auditory vesicle. 



