Mervin T. Sudler 411 



quite characteristic and aids greatly in identifying the gland at this 

 stage. It is a curved, elongated mass with an enlarged superior ceph- 

 alic end. The lower and smaller end runs parallel with the thyroid 

 rudiment until that bends abruptly and crosses the middle line. The 

 two structures do not come into contact at any point, however. 



In embryo CLXXA^, which is only slightly older, practically the same 

 condition prevails as in the model just described. The differences are 

 that the enlarged head is relatively smaller and the general curve of 

 the whole rudiment is not so pronounced, as shown in Fig. 12. The 

 small process that projects dorsally and laterally is still seen. 



In the model of embryo CXLIV of the sixth week the relative posi- 

 tion of the th3'-mus rudiment is lower and the division into different 

 areas and parts has disappeared. The head is barely recognizable as a 

 slight enlargement. They have approached the middle line at the 

 aboral end, although they still do not meet. In the model of embryo 

 XLIII the thymus is still lower so that now the thyroid rudiment is 

 above even the head of the thymus. The whole rudiment projects 

 beyond the thyroid and is nearer the middle line. In the model of the 

 oldest embryo studied, that of an embryo of the 7th week (embryo 

 XXII), the thymus rudiment has sunken relatively lower. It is in 

 contact with the thyroid rudiment along half of its upper surface and 

 the other end is free. In this lower free half the two rudiments 

 approach one another and meet in the middle line where the ends are 

 slightly swollen and bend ventrally. Apparently this is a beginning 

 of the folding of the thymus rudiment found in the adult organ. In 

 this model the thymus rudiment on the right side extends a little 

 higher than on the left side. The same condition is true in that of 

 embryo CXLIV. This will be referred to later in the discussion of 

 the lack of bilateral symmetry in the development of the thyroid. 



The Thyroid. 



The thyroid gland has proven of great interest to morphologists on 

 account of its origin and change of position and function. In the 

 tunicates it secretes an adhesive material used in the capture of the 

 food of the animal. In the higher mammals it has sunken to a lower 

 functional plane and its function has proven to be one of the puzzles 

 of the physiologist. Eemak discovered that it was of entodermal 

 origin. Stieda and Wolfler discovered the lateral rudiments, inde- 

 pendently, at the same time. Its origin is ably dealt with in Born's ^ 



=-Loc. cit. Es ist also ohne Zweifel, class der epitheliale Theil der Thyroidea 

 durch Verschmelzung zweier urspriiuglich raumlich getrennter und histologisch 



