The Pharyngeal Pouches in the Mammalia 223 



I shall at this stage treat of the factors which appear to be operative 

 in producing the present condition of the thj'mus. So far as I have 

 been able to determine, the modifications which the third pouch has 

 undergone are referable, in large part at least, to the operation of 

 purely mechanical factors. The pouch itself shows but little power of 

 active growth. Compared with the surrounding parts, it is relatively 

 passive. The first important factor is found in the unequal rate of 

 growth of the neck and of the ventral tubular process. The former 

 carries the roots of the large arteries from their original position imme- 

 diately under the pharyngeal pouch region to their definitive position 

 in the upper part of the thorax. The ventral extremity of the thymus, 

 it will be recalled, is from the start in close relation with the bases of 

 the carotid and aortic arteries, and as the latter become displaced to 

 successively lower levels, this portion of the thymus is carried down 

 with them. The elongation of the neck takes place, however, at a more 

 rapid rate than that of the thymus, and thus produces on the latter a 

 tension tending to carry it downwards. This, however, is prevented by 

 the fact that the dorsal extremity of the thymus is, during the same 

 period, relatively fixed in position by its attachment to the skin 

 and pharynx. The result of such conditions would be to produce a 

 strong tension in the intermediate connecting portion, which in conse- 

 quence would become drawn out in the form of a thin cord, similar to 

 that shown in the present stage (Fig. 21, Cv.C). This view is sup- 

 ported by the fact that the diameter of the cord is actually, and not 

 merely relatively, much less than in preceding stages. 



As already mentioned, the dorsal extremity of the thymus is rela- 

 tively fixed in position. This condition is readily explained when it is 

 recalled that this portion is attached to both the ectoderm and the 

 pharynx and also bears the voluminous carotid gland, the mere bulk 

 of which alone would hinder any ready displacement, even through such 

 a plastic medium as the surrounding mesenchyme. 



An additional factor, first pointed out by Kastschenko, is found in 

 the behavior of the hypoglossal nerve. The latter underlies the fundus 

 prsecervicalis. As this structure in harmony with the attached thymus 

 becomes displaced in the ventral direction, it comes in contact with the 

 nerve, which is also carried downwards with it until it forms a strong 

 angular flexure, over which the outer part of the fundus curves like a 

 hook over a cord. The elastic reaction of the nerve would naturally 

 form a strong hindrance to any ready ventral movement of the head of 



