The Pliaiyngeal Pouches in the Mammalia 225 



The cat of 23.1 mm. (jSTo. 466, Harvard collection) shows the thymus 

 entirely free from the carotid gland, that on the left side being separated 

 from the latter b}^ a considerable interval. At its thoracic portion the 

 organ is subdivided into numerous lobules. 



In the 33 mm.. pig (No. 74, Harvard collection) only the dorsal end 

 of the thymus was specially examined. It still forms a flattened plate 

 associated with the carotid gland and the lobules of the fundus prsecer- 

 vicalis. It is connected by the cervical connective Avith the now large 

 jind much lobed thoracic thymus. 



In the 33 mm. cat (JSTo. 500, Harvard collection. Fig. 68) the thymus 

 is entirely unconnected with the carotid gland. The dorsal extremity 

 of the cervical cord (Cv.C.) is placed immediately outside of the ventral 

 edge of the lateral wing of the thyroid. The cord is somewhat con- 

 voluted. It is continuous at its ventral extremity with the thoracic 

 thymus, which is much enlarged and sul^divided into numerous lobules. 

 The latter are solid and are composed of small-celled epithelial tissue. 

 I could observe no indications of the formation of true lymphoid tissue 

 in it. 



In a rabbit of 20 days the thoracic thymus is very large and on each 

 side it is subdivided into numerous lobules similar to those seen in the 

 cat last described. It is connected by the cervical cord with the carotid 

 gland, which is located close to the dorsal edge of the lateral wing of 

 the thyroid. 



(c") The Origin and Structure of the Carotid Gland. 



The carotid gland first appears in a 9 mm. pig (M^ of my series, 

 Fig. 36) as a mass of intertwined solid vesicles (C.GL), representing a 

 series of folds of the anterior wall of the third pharyngeal pouch. The 

 gland is therefore a purely epithelial structure of endodermal origin. 

 The carotid artery (Car.) lies immediately in front of it and shows 

 a small branch extending back toward the gland. 



In the 10 mm. pig (Figs. 7-8, C.Gl.) the carotid gland is somewhat 

 larger. As in the preceding stage, it is continuous with the peripheral 

 half of the anterior wall of the pouch and even projects a slight dis- 

 tance beyond its lateral margin (Fig. 8), It here meets the ectoderm 

 of the anterior wall of the third pharyngeal groove. Dorsally it pro- 

 jects a short distance above the upper margin of the pouch and partly 

 curves backwards over it. The carotid artery (Car.) gives off a small 

 branch, which, on reaching the gland, divides into numerous capillaries 

 which form a rich network interpenetrating it in all directions. Yen- 



