DEVELOPMENT OF SUPRAPERICARDIAL BODY 397 



extension downward of the pharyngeal epithelium. It is com- 

 posed of a thick stratified squamous epithelium and contains in 

 its center a small blind pit, continuous dorsally with the cavity 

 of the pharynx. The caudal, ventral portion of the diverticulum 

 fuses with the epithelium of one of the small dorsal tubules of 

 the gland, but there is no communication between their cavities. 

 The stratified epithelium of the diverticulum, as well as that 

 lining the pharynx, contains numerous mucous goblet cells scat- 

 tered through its entire thickness (fig. 16). 



The mesenchyma of the pharyngo-pericardial wall of the 

 earlier embryos has now developed into a fibrous connective 

 tissue. Underlying the pharyngeal epithelium the fibers are 

 collected into dense, compact bundles which project up under 

 the epithelium, forming broad square-topped papillae. Sur- 

 rounding the gland tubules, also, the fibers are relatively dense 

 as compared to the coarse-meshed intervening stratum. 



In the newborn fish, although the gland is actively secreting 

 mucus, typical cells distended with mucus are not numerous. 

 The tubules and vesicles still vary greatly in size. Some of the 

 smallest elements of the gland are cords of cells without a lumen, 

 in which the distended mucous cells are most frequently found. 

 The tubules and vesicles are lined with a single layer of cubical 

 or columnar cells. The lumina are either partially or com- 

 pletely filled with secretion, containing a few necrotic cells. 

 Mitotic figures are found in most of the tubules, but are espe- 

 cially numerous in the larger vesicles. The epithelial down- 

 growths of the pharynx, mentioned in the earlier description of 

 the newborn animal, are composed of stratified squamous epi- 

 thelium. They both contain mucous goblet cells and are very 

 similar to the diverticulum described in the 'pup,' except they 

 do not contain a true lumen which communicates with the cav- 

 ity of the pharynx. The larger downgrowth, located in the 

 cranial portion of the gland does contain a small blind cavity, 

 but it is formed by the fusion of several large globules of mucus 

 (fig. 17). 



The connective tissue in the caudal portion of the gland forms 

 a distinct sheath surrounding the tubules, which becomes con- 



