The Pharyngeal Pouches in the Mammalia 233 



The dorsal body of the pouch after the separation of the attached 

 fundus prfficervicalis from the skin forms the relatively inconspicuous 

 dorsal extremity of the thymus. It loses all trace of a lumen and thus 

 forms a solid epithelial plate wedged in between the carotid gland and 

 fundus prsecervicalis. 



The fully formed thymus is differentiated into three parts — a ventral, 

 thoracic thymus, an intermediate cervical cord and a dorsal plate to 

 which the carotid gland is attached. In the cat the connection of the 

 thymus with the carotid gland is interrupted in the later stages of 

 development. In the rabbit the two structures become disconnected by 

 the twenty-first day of development. 



The carotid gland is typically an ovoidal body located in the neck 

 close to the outer side of the lateral wing of the thyroid. Structurally 

 it is a reticulum of solid follicles, interpenetrated by a system of capil- 

 laries derived from the carotid artery. 



The sinus prsecervicalis, as a result of its passive deepening by the 

 outgrowth of surrounding parts, is transformed into a deep recess, the 

 fundus prsecervicalis. The latter is finally cut off from the ectoderm, 

 and, by retaining its connection with the dorsal plate of the thymus, 

 comes to lie at a considerable distance below the surface. Its inner 

 extremity forms for some time a vesicle, which enters into close relation 

 with the inferior ganglion of the vagus. 



In the cat the fundus apparently is early atrophied. In the pig, 

 however, it undergoes a strong proliferation, giving rise to a prominent, 

 irregularly convoluted mass closely associated with the carotid gland. 

 The peripheral lobe is separated from the remainder by the constriction 

 effected by the hypoglossal nerve. This portion represents the so-called 

 thymus superficialis of Kastschenko. 



In late stages of the rabbit all remains of the fundus prsecervicalis 

 have largely, if not entirely, disappeared. Only very doubtful traces of 

 it remain. 



The formation of the thymus as described in this paper is in harmony 

 with all the more recent observations. These prove that the organ is 

 of purely endodermal origin and that all but an insignificant portion 

 arises from the ventral diverticulum of the third pouch. 



With regard to the final lymphatic transformation of the thymus, I 

 can say little, owing to the fact that I did not have at my disposal a. 

 sufficient number of older stages to enable me to form any decided 

 opinion as to the process by which the change took place. In the rabbit 



