The Morphology of the Tadpole of Xenopus laevis. 247 
position such as that defined for the thymus by Sedgwick (p. 277), viz. 
close to the angle of the lower jaw (in the Anura behind the tympanic 
cavity and beneath the depressor mandibulse muscle"). I take it, there- 
fore, that it is these trabeculo-mandibular " thymus glands which are 
developed in Anura. In Xenopus, although relatively enormous in the 
tadpole, they have escaped my notice in the adult. 
Operculum. — The operculum on each side of the body is a ventro- 
kterally attached fold of skin which is broad in the middle and gradually 
tails out towards the posterior. Towards the anterior it becomes very 
narrow, and then its free anterior end is fused with the body-wall (Fig. 5, 
Fig. 5. — Bl, 2, and 3 1st, 2nd, and 3rd branchial bars ; CI and 2 1st and 2nd gill 
slits ; Gl and 2 1st and 2nd external gills ; Gpl and 2 1st and 2nd external gill pouches ; 
Ep epidermis ; Oc opercular cavity. 
OC). It is a theory, generally accepted I think, that the operculum 
represents an external gill growing backwards from the hyoid bar. Fig. 5 
shows that it only starts on a level with the beginning of the 2nd gill-slit 
and the end of the 1st gill-slit ; reference to Fig. 7 will show that this is 
far back from the hinder ends of hyoid. However, if the hyoid were 
continued straight backwards it would lie directly under the attachment of 
the operculum so that, also in Xenopus, the two lateral opercula may be 
taken to be hyoidean gills. 
