The Morphology of the Tadpole of Xenopus laevis. 
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of which are more ventral than the posterior. By these grooves the 
pharynx is w^idened in the manner shown by me in a previous paper. 
In connection with the first branchial slit, apparently opening into the 
opercular cavity, but really situated near the base of the external gill of 
the second branchial bar (as I shall show later under the heading 
Operculum "), there is another gland (Gl of Fig. 4 and Th2 of Fig. 2). 
These two glands, the one in connection with the mandibulo-hyoid 
cleft (Gl Fig. 1) and the other in connection with the 1st branchial cleft 
(Th2, Fig. 2 and Gl Fig. 4), are no doubt thymus glands. 
Fig. 3. — TG trabeculse cranii ; IJV internal jugular vein ; P palato-pterygoid bar ; 
ON optic nerve ; Sp hyobranchial slit ; Hy hyoid bar ; EJV external jugular vein ; 
Br 1st branchial bar attached to basi-branchial ; LV lingual vein ; OA carotid artery 
(external) ; Th thyroid gland ; Ly lymph spaces. 
The thyroid glands are present in the form of a pair of coiled tubules 
(Th of Fig. 3) which lie under the anterior connection, in front of I, 
Fig. 7, of the branchial bars to the basi-branchial. They do not open into 
the mouth in my youngest specimens. 
In Fig. 5 of my previous paper, already quoted, there is shown two 
glands on the outer side of each external jugular (EJ) ; the anterior is 
* Dreyer: Trans. Royal Soc. of South Africa, vol. iii., pt. 3, p. 353, 1913. 
