664 FRANCIS M. BALDWIN 



as the thymus bodies in that form. Maurer ('02), however, 

 points out that there is some doubt as to the homologies of these, 

 and says, p.. 132, "Ob sie der Thymus hoherer Wirbeltiere homo- 

 log sind, ob nur die dorsalen dafiir anzusprechen sind, die ven- 

 tralen aber als Epithelkorperchen zu deuten sind, ist nicht zu 

 entscheiden: ja es ist moglich, dass sie nur den Cyclostomen 

 zukommende Bildungen eigener Art darstellen, welche auf 

 gnathostome Wirbeltiere nicht iibergegangen sind." 



In the gnathostomes, structures which have been interpreted as 

 thymus bodies occur. Although arising from the endodermal 

 epithelium of various number of gill-pouches (usually from the 

 dorsal angle), these have been fairly well homologized in the 

 different groups of vertebrates (mammals perhaps excepted). 

 In the elasmobranchs, small thymus bodies develop from the dor- 

 sal angle of several (second to sixth and possibly the spiracle) vis- 

 ceral pouches. In the few teleosts in which the development has 

 been studied, and in the caecilians (Hypogeophis, Marcus, '08), 

 these develop from the second to the sixth pouches. In the uro- 

 deles, they arise from pouches one to five; the first two usually 

 degenerate, the last three persist to form the adult thymus. In 

 the anurans, only the first and second pouches form such bodies, 

 the second alone persisting to form the definitive structure in 

 the adult. 



Comparatively little is known of the development of the thy- 

 mus gland in the reptiles, but in the groups which have been 

 studied, some differences are found. In certain of the Lacertilia, 

 (Maurer ('99); Anguis, Gongylus, Lacerta, Saint-Remy, and 

 Prenant ('04), etc.) the thymus bodies develop from the dorsal 

 region of the second and third pouches, while in certain of the 

 ophidians (Coluber, Tropidontus), the third and fourth and 

 possibly the fifth pouch form a thymus lobule. (For further 

 details, see Maurer, '02, p. 136.) 



In the birds, a similar condition prevails. According to de 

 Meuron ('86) and Mall ('87), only the third pouch forms a thy- 

 mus body in the chick, while van Bemmelen ('86) and Verdun 

 ('98) believe the fourth also forms thymus elements, and the latter 

 author thinks the fifth pouch contributes as well. 



