PHARYNX AND AORTIC ARCHES 415 



traces of the postbranchial body. Neither Thompson nor 

 Doyen record the much larger anterior thymus and its enclosed 

 parathyreoid. The papers of Van Bemmelen and Thompson 

 are the most complete and supplement each other; my own re- 

 sults are in close accord with their descriptions. 



The turtle, in deriving its adult thymus and parathyreoid 

 glands from the third and fourth pouches, and not from the 

 first two, resembles the bird and mammals generally (Verdun). 

 When comparing mammals with other vertebrates, however, 

 one must not forget that the mammalian thymus and parathy- 

 reoid gland are ventral and dorsal outgrowths, respectively — an 

 arrangement just the reverse of that found in every other verte- 

 brate phylum. Maurer ('99 a, '06) has pointed out that the 

 thymus III of Lacerta encroaches ventrally on to the inter- 

 mediate pouch-tissue, thus possibly approaching the mammalian 

 condition. The same is certainly true of the thymus III of 

 turtles. There is, however, no change in the position of the 

 parathyreoid gland in either reptile. Maurer ('99 b, '06) em- 

 phasizes the absence of colloid in the postbranchial body of all 

 non-mammalian vertebrates, and considers the secretion to 

 appear first in Echidna. Whatever may be its chemical nature, 

 a colloid-like secretion is certainly present in the postbranchial 

 body of the turtle — perhaps another of the several adumbrations 

 of the mammalian condition to be found in this reptile. 



Aortic arches. The aortic arches are six in number. They 

 appear serially from before backward. The first two are tran- 

 sient vessels. The third persists as the internal carotid artery, 

 and the fourth as the systemic aortic arch. The fifth appears 

 soon after the fourth is completed. It becomes a fully developed 

 arch (fig. 6), which extends from the truncus arteriosus to the 

 dorsal aorta, and functions in embryos of from 6.5 mm. to 9 mm. 

 in length. The sixth arch begins as a bud from the fifth, spring- 

 ing from the latter above the fifth pouch (fig. 6) and running 

 ventrally behind the fifth pouch and lateral to the postbranchial 

 body. In the 6.6-mm. embryo the arch ends blindly; in an older 

 embryo of 8.4 mm. (fig. 11), it rejoins the fifth below the pouch 

 and becomes a complete functioning vessel. The sixth arch, 



