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Transactions of the Boyal Society of South Africa. 
the thyroid, the posterior the thymus. It seems therefore that the- 
anterior pair of thymus glands (those in connection with the mandibulo- 
hyoid cleft) do not develop in the adult. 
Besides these glands there is another pair of glands (Th Fig. 2) which 
are much larger and are situated near the posterior end of a deep groove 
running between the palato-pterygo-quadrate bar and the trabecula 
cranii ; the groove commences just behind the palatal attachment of the 
bar and is continued backwards below the pedicel attachment. The 
groove S, as shown in Fig. 2, may appear to be adventitiously formed by 
the two flaps growing downwards into the pharyngeal grooves ; further 
forwards, however [in Fig. 3 it has just " tailed out " in the edge of tha 
Fig. 4. — Lv = external jugular vein ; = 1st branchial bar; Gl thymus gland ; Gp, 
and Gpa 1st and 2nd external gill pouches. 
pharynx pointing in a direction between the pterygoid (p) and the 
trabecula (TC)] it can be seen to be a perfectly well-defined groove which 
is probably the vestige of a premandibular gill-slit. The almost spherical, 
ductless, compact gland produced as a proliferation of the epithelium of 
such a slit could then also be called thymus " gland. Now what I take to 
be the thymus glands of the adult are two yellowish, fatty-looking bodies 
situated in the floor of the mouth just behind the thyroid glands (the latter 
still recognizable as a pair of coiled tubules), i.e. exactly in the situations 
of the thymus glands of the 1st branchial slit. But the dorsal glands, i.e. 
those in connection with the " trabeculo-mandibular " slit w^ould have a. 
