The Pharyngeal Pouches in the Mammalia 231 



In the 33 mm. pig (Pigs. 34-35) the fundus shows a remarkable 

 increase in size and now forms a prominent, irregularly lobed mass 

 appendaged to the carotid gland (F. Pc). That portion (P. Pc. V.) 

 adjoining the vesicula has subdivided into several lobules, from which 

 the original vesicula itself is not clearly distinguishable. From this 

 part the thin band curves dorsally around the outer side of the carotid 

 gland, and at its upper extremity is continued into the hook-shaped 

 process, which has undergone a remarkable proliferation into a rela- 

 tively immense, much convoluted mass (F. Pc). The hypoglossal 

 nerve (XII) partly divides it on the left side (Fig. 34) into two 

 unequal lobes — a large outer and a smaller internal. On the right side 

 (Fig. 35) the division is complete, the outer being separated from the 

 inner by a thin plate of connective tissue. The outer, free portion 

 represents the so-called thymus superficialis of Kastchenko. (Fig. 35; 

 Th.S.). 



This division of fundus is evidently — as indeed Kastchenko first 

 pointed out — a result of the pressure produced by the hjrpoglossal 

 nerve. On the left side (Fig. 34), where the two divisions of the 

 prgecervical mass are united, this nerve lies in the constriction between 

 them immediately under the connecting cord. Where, as on the right 

 side (Fig. 35), the division is complete, the nerve lies entirely above 

 the prgecervical mass. These relations naturally suggest that the dis- 

 placement of the nerve itself has been the active cause in producing the 

 present condition of the structure. This view is further supported by 

 the relations between the two structures as observed in earlier stages. 

 In pigs of 10, 13 and 14 mm, the nerve, after descending behind the 

 sinus prgecervicalis, curves forward some distance below its ventral 

 margin (Fig. 40). In the 17 mm. pig it lies immediately under the 

 fundus close to where the latter meets the carotid gland (Figs. 45-46), 

 It has thus assumed, relative to the fundus, a more dorsal position — a 

 change associated with the ventral displacement of the thymus owing 

 to the elongation of the neck. Later, as these alterations in position 

 continue, the nerve produces an upward pressure on the fundus, and 

 thus causes it to assume an ascending course with its peripheral free 

 extremity hanging loosely, like a hook, over the nerve. This is the 

 condition observed in a 35 mm. pig (Fig. 31). This portion of the 

 fundus then undergoes a rapid proliferation, perhaps an indirect result 

 of the mechanical irritation produced by the nerve, and thus attains the 



