402 WALTER E. CAMP 



the gland, that, in some of the sections, the vesicles appear to 

 be suspended in the vascular network (fig. 17). The vessels 

 surrounding the gland drain ventrally into a plexus of larger 

 vessels lying just under the pericardium. This subpericardial 

 plexus drains laterally into the inferior jugular vein. I have 

 seen no muscle-whorls in any of the specimens examined. 



No striking differences are to be found in the circulation of 

 the adult from that described for the 'pup' and newborn. The 

 venae lymphaticae are larger and in some places their walls con- 

 tain lymphoid nodules of considerable size (fig. 18). 



DISCUSSION 



The original conception of van Bemmelen ('85) that the supra- 

 pericardial body in selachians represents a rudimentary branchial 

 pouch is probably correct. This conception is shared also by 

 Greil ('05) who describes in selachians a true anlage of a seventh 

 pouch. I was unable to find the early anlage of the seventh 

 pouch as described by him. In the embryos of much older 

 stages, 80-95 mm. in length (S. C. 60 and H. E. C. 1882), there 

 is a distinct outpouching of the pharyngeal wall, both dorsal 

 and ventral, on either side, and exactly in the place where one 

 would expect a seventh pair of pouches to arise (fig. 6). The 

 suprapericardial body arises from the medial wall of the ventral 

 outpouching. In the younger stages, particularly at the time 

 of appearance of the gland or shortly before, no such outpouch- 

 ings are to be found. In one embryo of 28 mm. (S. C. 6) the 

 left suprapericardial body forms a single diverticulum which 

 extends ventrally almost to the pericardium (fig. 19). It is 

 widely open to the cavity of the pharynx and is lined by a single 

 layer of tall columnar cells, very similar to those of the gill- 

 pouches. This gland resembles very closely in its appearance 

 a developing pouch. There is no corresponding structure on 

 the right side nor any indication of a dorsal outpouching above 

 it. From the position of the gland — medial and slightly caudal 

 to the sixth pouch — and from its mode of development as an 

 elongated diverticulum from the pharynx which resembles (very 

 strikingly in one embryo, S. C. 6) a developing pouch, I believe 



