32 CHARLES EUGENE JOHNSON 



that such is not the case. In embryos of nine days the cellular 

 mass has grown very considerably larger and has become more 

 compact structurally. It is very irregular in form and its posi- 

 tion is changed so that it now lies just dorsal to the thyreoid- 

 parathyreoid group, but it extends somewhat beyond this group 

 posteriorly and encircles the fourth aortic arch from the medial, 

 posterior, and lateral sides; a portion of it also has grown in be- 

 tween the thyreoid and parathyreoids. At twelve days the dis- 

 position of the mass is in general much the same as at the ninth 

 day and there is no marked increase in extent, but the mass as a 

 whole is more clearly delimited. The greater part of it lies in 

 close contact with the dorsal surface of the thyreoid, with no 

 intervening connective tissue layer. There are no ramifications 

 entering the thyreoid gland, however, and the same is true for 

 the parathyreoids, but these two are now enclosed in a common 

 connective tissue sheath and are not in direct contact with the 

 postbranchial mass. The posterior portion surrounds the 

 brachiocephalic trunk near the origin of the subclavian artery. 

 The gland contains as yet no indications of epithelial vesicles. 



Because of its place relations to the organs under consideration, 

 the carotid gland is to be mentioned. This gland becomes clearly 

 recognizable in grebe embryos of about eight and a half days. It 

 then lies at the dorsolateral side of the carotid artery, just 

 cephalad of its junction with the subclavian. Its posterior por- 

 tion is in close proximity to the arterial wall, but anteriorly the 

 gland extends obliquely laterad, away from the vessel, and lies 

 near the- dorsolateral surface of the thyreoid, with the parathy- 

 reoid group bounding it laterally (fig. 9). On the left side, by 

 the presence of the postbranchial body medially and dorsally, 

 the gland becomes quite surrounded. 



THE ORIGIN OF THE POSTBRANCHIAL BODY 



In an investigation of the early stages of the gill pouches in 

 the duck, Rabl ('07) has described what he recognizes as a sixth 

 visceral pouch, represented by a diverticulum appearing in em- 

 bryos of about four and a half to five days, on the caudo-medial 

 wall of the pharyngeal evagination which is generally held to be 



