The Pharyngeal Pouches in the Mammalia 221 



In the 9.7 mm. cat (No. 466, Harvard seiies, Figs, 61, 62) the 

 ventral diverticulum (Y.D.3) of the third pharyngeal pouch is slightly 

 more elongated at its ventro-internal angle. In other respects it is 

 essentially similar to the stage last described. 



The pig of 10 mm. (No. 401, Harvard series, Figs. 4-8) shows clearly 

 the initial steps in the formation of the thymus duct. The ventro- 

 internal angle of the pouch is now clearly elongated in a ventro-mesial 

 direction and ends in an acute angle which is wedged in the angle 

 between the roots of the carotid and aortic arches (Fig, 7). The down- 

 growths of the two sides are not quite symmetrical, the right being 

 slightly larger and ending in a more acute angle than that on the 

 opposite side. Laterally the lower part of the pouch has separated 

 from the ectoderm, leaving only its dorsal half in contact with the 

 latter. Below the point of contact the lateral border turns obliquely 

 inwards and downwards and is continued into the ventro-lateral edge 

 of the thymus downgrowth. 



On the dorsal side the peripheral portion of the pouch projects 

 slightly above the upper end of the verschlussmemhran as a dorsal 

 diverticulum (Figs. 4-8, D.A.3). 



The carotid gland (Figs. 7-8, C.Gl.) lies closely adpressed against 

 the anterior wall of the pouch and on its dorsal side projects slightly 

 above the upper margin of the latter (Fig. 8). We shall defer further 

 consideration of the gland until later. 



In the 12 mm. pig (No. 518, Harvard series. Figs. 9-11, 13) the 

 ventral diverticulum (V.D.3) is further elongated. In addition to the 

 ventro-internal direction which it took in the earlier stage, it now 

 shows a pronounced anterior trend, an effect of the ventral rotation of 

 the adjacent part of the pharynx. At this time it has acquired a 

 distinct tubular form. 



The union of the pouch to the ectoderm is limited to a short stretch 

 immediately below its dorsal apex. The rest of its lateral border is 

 free and is continued ventrally into the outer edge of the tubular down- 

 growth. 



The part connecting the pouch with the pharynx is considerably more 

 constricted than in the preceding stage. 



The tubular downgrowth is still further elongated in a 14 mm. pig 

 (No. 65, Harvard collection. Figs. 14-16, Thy.). Its blind ventral 

 extremity lies a slight distance below the level of the pericardio-cervical 



