122 EDWARD MCCRADY, JR. 



Seesel's pouch is just another name for the anterior end of 

 the foregut, and as the pharyngeal bursa in the opossum is 

 located at this point, the two may be said in a sense to coin- 

 cide; but that this is nothing more than a coincidence, seems 

 clear from the fact that in man the obviously homologous 

 bursa lies some distance caudad of Seesel's pouch. Of course, 

 Rathke's pouch is on the opposite side of the pharyngeal 

 membrane and has no connection with either of them. 



In the same year that Selenka wrote his description, 

 Schwaback (1887) erroneously interpreted the bursa as "a 

 crypt connected with the formation of the pharyngeal tonsil. ' ' 

 Selenka was correct in attributing a causal role to the noto- 

 chord, but was probably mistaken in supposing that the duct 

 of the bursa represents the notochordal canal. A recent study 

 of this structure in man was made by G. C. Huber ('12), to 

 whose paper I am indebted for the references to the other 

 work on man. 



The pharyngeal bursa in the opossum differs from that in 

 man in being formed as a result of a terminal instead of a 

 sub-terminal contact of the notochord with the pharyngeal 

 epithelium; but I do not doubt that it has the same morpho- 

 logical significance. 



The dorsal pancreatic diverticulum. The dorsal anlage of 

 the pancreas arises as a long, finger-like diverticulum from 

 the middle of the roof of what turns out to be the duodenal 

 portion of the foregut, though it would really be a prolepsis 

 to speak of a duodenal portion at this time as there is no 

 visible indication of the stomach as yet. The pancreatic di- 

 verticulum is slightly caudal to that of the liver as well as 

 being dorsal instead of ventral. It is shown in figure 39, J. 

 In the general reconstruction it is partly hidden behind the 

 omphalomesenteric and umbilical veins. 



It is interesting that in the opossum the pancreatic anlage 

 is present before there is any recognizable stomach rudiment. 

 In man and in the pig it does not appear until the gastric 

 enlargement is quite distinct. This difference may be corre- 

 lated with the fact that in the opossum the liver and pan- 

 creas are both needed for the digestion and utilization of milk 



