PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 655 



die bei Anuren die Thymus hervorgehen lasst, sich sehr frlih 

 mit der ersten Knospe riickbildet." Later, in Hertwig's Hand- 

 buch, ('02) Maurer reviews briefly the development and position 

 of the thymus bodies in Triton and Siredon, in which he corrects 

 one or two points of his earlier work; these are noted below in 

 connection with Drtiner's discussion. 



Contrary to Maurer's description of the development of the 

 thymus in the other urodeles, Livini ('02) finds the permanent 

 thymus in Salamandrina to be the fifth larval body, formed from 

 the fifth gill-pouch, and states that the bodies of the third and 

 fourth as well as the first and second pouches degenerate early 

 without leaving a trace. 



In his work on the musculature and gill-region of the urodeles, 

 Driiner ('04), touches upon the developmental relationships of the 

 thymus in Siredon. He found, as did Maurer, five thymus ele- 

 ments arising from the dorsal region of all five gill-pouches; of 

 these the first two degenerate early, and the last three form the 

 permanent thj^mus. In one or two cases he found a small fol- 

 licular-like body persisting in the relative position of the first 

 thymus body, which he interprets as a remnant of the same. Ac- 

 cording to him also, not only entoderm from the gill-pouches is 

 concerned in the formation of the five primitive bodies, but ecto- 

 derm also contributed to their formation. On the other hand, 

 he agrees with Maurer ('02), in locating the first body as lateral 

 to the facial, rather than the Gasserian ganglion, and the second 

 body lateral to the glossopharyngeal instead of the facial, as was 

 stated by Maurer ('88). He also locates the third and fourth 

 bodies with reference to the lateral branches of the branchial 

 nerves of the vagus, ralther than to the ganglion itself. 



Marcus ('08, p. 737) found th3niius thickenings on the dorsal 

 portions of six gill-pouches of Hypogeophis, those on the first and 

 sixth, however, were rudimentary and degenerated early, while 

 the other four pouches formed thymus bodies, which, after sepa- 

 rating from the pharyngeal epithelium, became in the adult a four- 

 lobed thymus gland, the lobes of which were separated by thin 

 strands of connective tissue. 



Of the two more recent workers in the field of the histogenesis 



