K. T. Bell 35 
trachea, thyroid, and lower part of the larynx. A slight narrowing at 
the junction of the upper and middle thirds indicates the position of the 
intermediary cord. The antero-gorsal parts the thymus superficialis, is 
rounded in cross-section, tapertrfg to a point behind. Its anterior end is 
about 7 mm. in diameter and loops around the twelfth nerve as in the 
earliest stages. A lobule hangs over ventral to the nerve, a thin cord 
being dorsal to it. The posterior end of the thymus superficialis extends 
to the cricoid cartilage dorsal to the postero-ventral part of the gland. 
It is united to this part of the gland by a very delicate band of thymic 
tissue. I did not find the nodulus thymicus at full term. It has either 
moved away from the head or degenerated. 
The duct of the thymus is the lumen of the third gill pouch. A 
glance at the Text Figures will show its development. It is broken up 
into segments and finally obliterated. I could find no traces of it at 
3.7 em. or later. On the theory of the exclusively endodermal origin of 
the thymus, I cannot explain the absence of a lumen in the head at 
18 mm. and its presence at 20 mm. and 27 mm. unless it be due to 
individual variation in different specimens. 
It appears from the foregoing that in the pig the exclusively endo- 
dermal origin of the thymus from the third gill pouch is probable, but 
a slight participation by the ectoderm has not been satisfactorily excluded. 
Kastschenko’s conclusions, however, as to the ectodermal origin of the 
thymus are unwarranted by his recorded observations. Prenant, after 
his careful work (on the sheep), was not sure that a small mass of ecto- 
derm did not enter into the formation of the head. Practically all other 
investigators of this problem maintain that the ectoderm takes no part 
in the formation of the thymus. The epithelial body (nodulus thymicus) 
developing in connection with the head of the thymus from the third 
gill pouch does not form the carotid gland. Kastschenko’s description of 
the origin of the carotid gland in mammals from the adventitia of the 
internal carotid is now accepted by the majority of anatomists, and it 
therefore has nothing to do with the thymus in origin. 
THE HIstToLocy or THE FuLLY-ForMED THYMUS. 
Before taking up the histogenesis, I shall briefly consider the histology 
of the gland as shown in a 24-cm. embryo. At this stage the gland may 
be regarded as fully formed. As is well known, the thymic lobule con- 
sists of a cortical and a medullary portion,—the medulla of all the 
lobules being united by the medullary cord. The cortex consists of a 
delicate reticulum with its spaces well filled by cells, usually lymphocytes. 
The reticulum may be regarded as composed of small branched anastom- 
