36 CHARLES EUGENE JOHNSON 



less obscured. Since even where these last two become clearly 

 differentiated as separate diverticula, the period during which they 

 remain so is comparatively brief, this phase in their development 

 may readily escape observation. 



SUMMARY 



The thymus in the pied-billed grebe is derived from the third 

 and fourth gill pouches. In each pouch the anlage arises from 

 a prohferating area beginning in the anterior wall and continu- 

 ing in the dorsolateral wall to the posterior end of the pouch. 

 The larger part of the thymus is contributed by the third pouch. 

 The much smaller derivative of the fourth pouch becomes fused 

 with the caudal portion of the thymus III at about the ninth 

 day, and at twelve days the mass resulting from the fusion has 

 greatly increased in size and its connection with the remaining 

 part of the thymus has been reduced to a narrow band. 



A parathyreoid body is formed from each of the third and fourth 

 gill pouches. Their anlagen, which appear soon after those of the 

 thymus, arise as thickenings of the medial walls of the pouches, 

 but eventually all of each pouch, except the thymus portion, 

 becomes converted into the parathyreoid body. Soon after the 

 ninth day the parathyreoids become separated from the thymus 

 bodies and assume a position upon the dorsolateral side of the 

 thyreoid, the larger parathyreoid III being medial, the smaller 

 parathyreoid IV, lateral. At twelve days they still have the 

 same position, and are closely apposed and enclosed in a com- 

 mon sheath of connective tissue. 



Of the postbranchial bodies the left alone persists, the right 

 having disappeared by about the seventh day. At twelve days 

 the persisting body is a relatively large irregular mass lying 

 upon the dorsal side of the thyreoid, its posterior portion encirc- 

 ling the brachiocephalic trunk. Epithelial vesicles are as yet 

 not formed. 



The postbranchial body arises from the median of two divertic- 

 ula into which the so-called fifth visceral pouch bf birds becomes 

 divided, and which Rabl in the duck has interpreted as a sixth 



