408 KALPH FAUST SHANER 



to 7 mm. — the pouches are deep and fish-like, each with a 

 very narrow sHt-shaped lumen. ^ 



Behind the fourth pouch, in the angle between it and the 

 pharynx, there develops a diverticulum of a different nature. 

 It appears first in a 5.5-mm. embryo (figs. 3, 4), and is pointed 

 more caudally and medially than the true pouch outgrowths. 

 In a 5.6-mm. embryo (fig. 5), it ends in a flattened expanded 

 knob of considerable size. The diverticulum is clearly a post- 

 branchial body. 



On the postbranchial body there now appears a secondary 

 outgrowth (fig. 6), which points due laterally and which fuses 

 with a slight indentation of the ectoderm. A branchial placode 

 is placed behind the outgrowth, and the fifth and six aortic 

 arches lie before and behind it, respectively. In short, the 

 secondary outgrowth is a true fifth pouch, which differs from 

 the four anterior ones in arising after the formation of the post- 

 branchial body, and in growing from this body rather than 

 from the pharynx proper. 



Such a full development of the fifth pouch and the postbranch- 

 ial body enables one to make at least a partial interpretation 

 of the postbranchial body. It is not a pouch, for a pouch de- 

 velops from it: it never has those relations to nerves, branchial 

 placodes, and aortic arches which characterize a true pouch. 

 Neither is the postbranchial body an outgrowth of a pouch, in 

 the sense that the thymus and parathyreoid glands are, for it 

 grows out directly from the pharyngeal wall, and precedes the 

 fifth pouch, the only one from which it could be derived. The 

 true pouch derivatives do not appear until much later, and when 

 they do, the postbranchial body is not in series with them. The 

 postbranchial body is better considered as a caudal prolonga- 

 tion of the pouch-forming area — a remnant out of which the 

 fifth pouch develops, and from which a sixth would spring, if 

 such were to appear. 



2 The length of the embryos used in this paper is the head-breech dimension. 

 But turtle embryos coil up at very uneven rates, so that such measurements give 

 only an approximate idea of their age. 



