DEVELOPMENT OF SUPRAPERICARDIAL BODY 389 



mass of tubules and vesicles located in the extreme dorsal region, 

 just cranial to the pharyngeal diverticulum. The duct is short 

 and at its ventral extremity divides into two branches, one run- 

 ning caudally to end in a small tubule which terminates blindly, 

 and the other running cranially to open into a rather large 

 vesicle. This vesicle and tubule, when followed through the 

 sections, are found to be entirely independent of the remaining 

 portion of the gland. The duct and its connections has been 

 modelled and are shown in figure 25. The lumen of the duct 

 is small and opens directly into the cavity of the pharynx at the 

 bottom of a deep longitudinal groove in the pharyngeal epi- 

 thelium. 



HISTOLOGY AND HISTOGENESIS 



At the time of its appearance the suprapericardial body is a 

 simple saccular evagination of the pharyngeal epithelium. The 

 pharynx at this time is lined by a single layer of cells, cubical in 

 shape near the median line, but gradually becoming columnar 

 at the attachment of the gill-pouches. The suprapericardial 

 body arises just lateral to the region where this transition occurs 

 (fig. 1). The cells forming the anlage of the gland are decidedly 

 columnar in shape (fig. 8). Their nuclei are located in their 

 basal portions; are elongated oval in outline, and show a definite 

 chromatic reticulum. The bulk of the chromatin is near the 

 nuclear membrane, but a few masses are found scattered through- 

 out the nucleus. The nuclei of the cells forming the medial 

 wall of the outpouching at its union with the pharynx are very 

 much elongated and closely crowded together, due to a slight 

 twisting of the mass toward the median line. A definite mem- 

 brana propria separates the epithelial cells from the underlying 

 mesenchyma. 



At 24.7 mm. (H. E. C. 1492) the saccular outpouching has 

 become converted into an elongated tubule extending from the 

 pharyngeal epithelium to the parietal pericardium. The cavity 

 of the tubule communicates widely with the pharynx and is lined 

 by a single layer of low columnar cells. The tubule, although 

 short, is very tortuous, and, at the points of flexure, the cells 

 are very much elongated and deeply stained. A little later in 



