DEVELOPMENT OF SUPRAPERICARDIAL BODY 377 



Figure 2 is from a wax reconstruction of the pharynx of an 

 embryo 20.6 mm. long (H. E. C. 1494), showing the position of 

 the gland at the time of its appearance. This embryo is No. 

 28a of Scammon's Normal-plate and corresponds approximately 

 with Balfour's stage N. The pharynx is rather flat and arches 

 dorso-ventrally. Its cranial extremity is broad and relatively 

 large but rapidly tapers caudally into the flat and narrow 

 oesophagus. All of the gill-pouches are formed. They have a 

 rather wide origin from the lateral margins of the pharynx, and 

 project outward at almost right angles to the main axis of the 

 embryo. Their dorsal extremities are bent slightly caudalward 

 and are much more pronounced than the ventral ones, which 

 are small, narrow and crowded close to the wall of the pharynx. 

 All of the pouches with the exception of the first are open along 

 their ventral and lateral margins. The thyreoid is represented 

 by a single median elongated mass at about the level of the 

 second pair of gill-pouches. It is in close proximity to the 

 ventral wall of the pharynx from which it has but recently lost 

 its connection. The anlagen of the thymus have not made their 

 appearance. 



The suprapericardial body at this stage is found only on the 

 left side. It arises from the pharynx slightly caudal to the 

 origin of the ventral extremity of the sixth gill-pouch. It is 

 located just medial to a continuation of the line joining the 

 origins of the ventral extremities of the pouches. The epi- 

 thelium forming the gland is slightly thickened and evaginated 

 into the mesenchyma to about half the distance to the peri- 

 cardium which surrounds the atrium (fig. 1). 



In slightly older embryos (24.7 mm. H. E. C. 1492) the supra- 

 pericardial body has about the same relative position as in the 

 specimen just described. At this stage also the body is found 

 only on the left side. The small plate of epithelium has become 

 converted into a digitiform tubule which reaches almost to the 

 pericardium. The tubule is slightly constricted at the point of 

 its connection with the pharyngeal epithelium. It contains a 

 very small lumen which communicates directly with the cavity 

 of the pharynx. 



JOURNAL OF MORPHOLOGY, VOL. 28, NO. 2 



