228 Henry Fox 



and anterior cervical prominence converge. The fourth pharyngeal 

 arch^ which is located at the inner extremity of the fundus, is thus 

 j)artly hidden in lateral view. 



The fundus has much the same features as hitherto. Its inner ex- 

 tremity, while still connected at one point with the fourth pouch (Fig. 

 62, Ph. P. 4), is prolonged above the latter and forms a slight dilata- 

 tion situated a short distance back of the dorsal edge of the third pouch 

 (Fig. 62, V.Pc). This free, dilated portion is evidently to be. com- 

 pared with the vesicula prsecervicalis (= vesicula thymicus of Kast- 

 schenko), which will be more fully considered in the descriptions of the 



pig- 



The 10 mm. pig (Figs. 5-8) shows the sinus in a condition somewhat 

 intermediate between the two last described. On the right side the 

 fundus and the fourth pharyngeal pouch are still connected with each 

 other, but on the opposite side they are entirely separate and are 

 removed from each other by a considerable interval, which is largely 

 occupied by the aortic arch proper (Ao.). The large size of the latter 

 suggests that it may have been an active agent in effecting the separa- 

 tion of the pouch from the fundus. Thus, on the side where the two 

 structures are still connected, the ventral side of the artery is closely 

 pressed against the connecting part and is evidently exerting a pressure 

 upon it which would tend to effect its separation. As we shall see, this 

 soon takes place. 



The inner extremity of the fundus is formed by a narrow, obliquely 

 vertical groove — the fourth pharyngeal groove (Fig. 8, Ph. G.4). Where 

 the fourth pouch retains its connection, it is confined to the ventro- 

 internal angle of the fundus. From this point the remainder of the 

 groove ascends diagonally forwards and at its dorsal extremity meets the 

 corresponding part of the third groove (Fig. 7, Ph.G.3). There is thus 

 included between the two a triangular convexity which represents the 

 fourth pharyngeal arch. The third groove in its entire extent is con- 

 nected with the underlying pouch. The latter is not joined to the 

 bottom of the groove, but to its anterior wall, the lateral margin of the 

 pouch reaching some distance beyond the deep part of the groove (Figs. 

 7-8). 



On the side where it is no longer joined to the fourth pouch the 

 blind, inner end of the fundus is turned obliquely backwards and comes 

 into close relation with the inferior ganglion of the vagus. This part 

 we shall hereafter designate the vesicula prascervicalis. In my estima- 



