714 J. F. GUDERNATSCH 



in the peripheral. In some instances, as in Cynoscion, rather well 

 developed central portions are found which can be recognized by 

 the naked eye. But even in these cases numerous follicles lie well 

 separated from the main body. Thus we have all transitional 

 stages from a perfect dispersion of the follicles to a rather compact 

 union of them. It is possible that further investigations may 

 show cases in which the organ is still more compact than in those 

 thus far examined, and so present a structure similar to that in 

 higher vertebrates. Judging from my observations, however, 

 this does not seem probable, and at present I am inclined to re- 

 gard the conditions found in Sarda {vide Special Part) as the limit 

 of compactness in the series. 



The cephalad and caudad extensions of the thyreoid gland vary 

 very much. In general, it might be said that a spreading out to- 

 ward the tip of the tongue takes place in all cases, while towards 

 the heart the distribution is not so uniform. Far cephalad of the 

 first aortic bifurcation we find single follicles scattered below the 

 hyoid bones. The caudal limit of the thyreoid gland usually lies 

 between the second and third aortic branches, or at the third. 

 Rarely does it go beyond this point, and if so, with a few excep- 

 tions, only scattered folhcles are found in the posterior region. 

 Thus we find an accumulation of the glandular elements around 

 the anterior part of the ventral aorta, with follicles scattered 

 towards the head and the heart. 



The organ decreases in mass in an anterior-posterior direction. 

 In one instance, Siphostoma, just the reverse is the case. 



The embryonic center from which the thyreoid starts to grow 

 lies between the first and second gill branches, this place in the 

 adult animal is near the aortic bifurcation, and in manj^ cases 

 we find the main part of the organ in this region; while in other 

 cases, Cynoscion and Tautogolabrus, it is exclusively there. 

 From the region of the aortic bifurcation the thyreoid elements 

 travel so as to occupy the different positions which are described 

 in the special part of this paper. The migration of thyreoid 

 follicles occurs more particularly in the direction of the heart, 

 although a cephalad migration is decidedly pronounced. The 

 development of the hyoid bones obstructs the anterior spreading 



