162 THE STRUCTURE OF MAN 
been developed out of the posterior part of the degenerating 
sublingua. 
The study of Ontogeny has up to the present thrown no 
light on the sublingua. | 
Before quitting the tongue the papille foliate should be 
mentioned. These, in Mammals, take the form of localised — 
systems of lamellee, situated on the postero-lateral tongue border, 
and having their epithelium thrown into a series of flask-shaped 
depressions. In Man these papille vary much in form and size, 
and since they are occasionally represented by but mere traces 
they are evidently undergoing reduction. 
THYROID AND THYMUS 
These two organs are developmentally related to the pharyn- 
geal region. 
The thyroid gland, in all Mammals in which it has been 
examined, arises from two ventral outgrowths, one of which is 
paired and the other unpaired. 
The unpaired constituent is closely connected ontogenetically 
with the tongue which, during development, bridges over the 
floor of the buccal cavity, enclosing a space, the wall of which 
becomes changed into an epithelial vesicle. This is the unpaired 
or median thyroid gland, and it for a time remains in com- 
munication by means of its duct (the ductus thyroglossus) with 
the posterior surface of the tongue, at its base of attachment. 
When this duct closes, its orifice may become converted into the 
so-called foramen ccecum of the adult, and therefore belongs to 
the class of vestigial structures. The duct itself, as His has 
shown, may often be retained in the adult for a length of 24 or 
more centimetres. Its existence explains the fact that the so- 
called middle lobe of the thyroid gland is occasionally prolonged 
upwards into a process, which often becomes constricted so as 
to form a series of from two to four longitudinally recurrent 
vesicles (burse supra hyoidea and preehyoidea). 
The paired portions, or the lateral lobes, of the thyroid gland 
arise at the region of extreme posterior differentiation of the 
visceral skeleton, by constriction of the primary floor of the 
pharynx, near the laryngeal orifice. We have thus, here again, a 
structure of epithelial origin. Ata later stage the lateral and 
median portions of the thyroid gland become approximated. 
