240 Henry Fox 



lateral wing of the th3^roicl. As in earlier stages, it is readily distin- 

 guishable from the latter by its different histological structure. 



Review and Comparisons. 



The fourth pharyngeal pouch resembles the third in producing two 

 distinct structures, the lateral thyroid and the "glandule thyroidienne." 



The lateral thyroid is formed by the elongation of the ventral diver- 

 ticulum. This at first is perfectly continuous with the side of the 

 pharynx, but the connecting part early becomes constricted, assumes the 

 form of a solid cord and subsequently separates from the pharynx. The 

 remaining ventral portion forms a piriform vesicle, which soon becomes 

 solid and later by baclrward growth of the median thyroid becomes 

 embedded in the lateral wings of the latter. 



The dorsal body of the pouch early loses its connection with the 

 ectoderm and undergoes partial atrophy. A considerable portion, how- 

 ever, is transformed into the "glandule thyroidienne'' of Prenant. In 

 the cat I have not been able to trace the history of this structure, but 

 Verdun, who examined a large series in this type, asserts that along 

 with the lateral thyroid it becomes embedded in the lateral lobe of the 

 median thyroid. In the pig it persists for a considerable period, but 

 never forms any connection with the median thyroid. 



The results of my study of the fourth pouch and its derivatives are, 

 as a whole, corroborative of previous investigations. The main facts in 

 its development had been ably presented by Kastschenko, although he 

 overlooked the "glandule thyroidienne" which was described by Prenant. 

 The latter gives a full account of the microscopic structure of the organ 

 and distinctly asserts its homology with the carotid gland of the pre- 

 ceding pouch. 



A^erdun derives from the fourth pouch another structure which he 

 terms "th3^mus IV" on account of its supposed homology with the 

 thymus of the preceding pouch. This, he states, arises as a "diverticule 

 externe et ventral." In all the examples examined by me I have noticed 

 nothing to suggest this structure. Verdun describes it most fully in 

 the case of the cat, but states that it is only exceptionally, and then 

 only slightly, developed in the rabbit. In the camel and ox he finds it 

 doubtfully represented by certain lobules associated with the "glandule 

 thyroidienne." In the other forms — man, mole, opussum, dog, pig, 

 sheep — ^he gives no very convincing evidence of its presence. In view 

 of these facts, i. e., its exceptional presence in one form and its doubtful 



