DEVELOPMENT OF SUPRAPERICARDIAL BODY 



405 



thyreoid. The duct opens directly into the oraJ cavity, but 

 posteriorly it is closed and is united to the thyreoid only by 

 connective tissue. 



The suprapericardial body in its development and structure 

 is intermediate in type between the ordinary branching gland 

 and the follicular ductless gland. It arises as an evagination 

 of the epithelium which very soon develops into a solid connect- 

 ing-stalk and a blind expanded portion. A part of the connect- 





V 



# 



Fig. 20 Diagrams showing three corresponding stages in the development of 

 (A) a typical branching gland in selachians; (B) the suprapericardial bod}-; and 

 (C) a closed-follicular gland (thyreoid). 



ing-stalk forms a large diverticulum which opens into the pharynx, 

 and is entirely separate from the rest of the gland. The remain- 

 ing part of the connecting-stalk and the blind expanded portion 

 of the gland develop into large irregular vesicles, the most of 

 which intercommunicate. A small part of the gland near its 

 point of origin becomes secondarily connected to the pharynx 

 by a true duct (fig. 20, B, 1, 2,3). 



The glandular epithelium of the portion of the gland having 

 a duct is identical in structure and appearance with that of the 



