300 CHARLES EUGENE JOHNSON 



there is formed a persisting parathyreoid body. The persisting 

 thymus is developed from the fourth and fifth pouches. In 

 Coluber, moreover, the fourth pouch gives origm to a persisting 

 parathyreoid, while a very rudimentary transitory body of this 

 kind, as a rule, is formed by the fifth pouch. In Tropidonotus, 

 on the other hand, the fifth pouch only exceptionally gives rise 

 to a parathjTcoid, which likewise is of transitory nature. The 

 right ultimobranchial evagination does not disappear, as in the 

 lizards, but, like the left, undergoes progressive development 

 into a glandular organ. The two are symmetrically situated in 

 Coluber, but in Tropidonotus the position of the right one is 

 somewhat variable. 



For the turtle group very little work appears to have been done 

 in connection with the branchial derivatives. A brief account 

 by van Bemmelen ('93) has reference to Chelonia \dridis. Ac- 

 cording to this account, the earher phases of the development of 

 the gill pouches correspond with the conditions in lizards and 

 in snakes, but in later stages there is greater similarity with the 

 processes in birds than with those of reptiles. Five pairs of 

 visceral pouches are recognized, of which the first three become 

 perforate, as does probably the fourth pair also. From the 

 second pouch arises an epithehal bud which develops into the 

 anterior lobe of the thymus; but the pouch itself becomes, as in 

 snakes, an isolated vesicle which is destined to disappear, as in 

 the case of the corresponding pouch in birds. The third pouch 

 becomes an expanded epithelial vesicle provided with numerous 

 secondary evaginations. It separates from the epidermal and 

 the pharyngeal epithelium, and the secondary evaginations 

 give rise to the thymous tissue, in the midst of which the central 

 epithelial cyst persists as a homologue of the 'carotid body' 

 of lizards. The fourth and fifth visceral pouches arise simul- 

 taneously with the 'suprapericardial' (ultimobranchial) evagina- 

 tion, from a lateral 'blinddarmformigen Falte' at the caudal end 

 of the pharynx (recessus praecervicahs), in the same way as in 

 snakes. These outpouchings soon become separated from the 

 pharynx and form a complex of three connected vesicles. If 

 these vesicles, says van Bemmelen, in their further development 



