CARCINOMA OF THE THYROID IN SALMONOID FISHES. 375 



of several layers of cubical cells. At this time no cartilage is differentiated, but the 

 hyoid arch is well developed and the true gill arches are indicated, the brain vesicles 

 and mouth opening are present, while the only macroscopic vessels of the region are 

 the S-shaped heart tube and the two hyoid arteries. 



The primary evagination lies close but not attached to the arterial wall, from which 

 it is separated by connective tissue. At 32 days it has become spherical, has a lumen 

 of its own, its epithelium is single layered and is connected with the mother cells only 

 by a slender pedicle. (Fig. 8.) At 35 days it has completely separated (fig. 9), is 

 about 0.04 millimeter in diameter, and lies exactly in the fork of the heart tube, which 

 is 0.1 millimeter in diameter before the bifurcation. The four pairs of true gill arteries 

 are now visible. 



During the next six days the vesicle becomes somewhat elongated and changes its 

 position. It now measures about 0.03 by 0.06 millimeter and has been pressed back 

 of the bifurcation and lies ventral to the gill artery trunk, or ventral aorta (fig. 10). 

 Colloid is now present and the vesicle begins to put out buds which rapidly develop a 

 lumen filled with colloid and quickly separate. These multiplying buds grow around 

 the aorta to the dorsal side and keep pace with its increase in length until, at jj days 

 or 3 weeks after hatching, that portion of the aortic trunk from the last branchial 

 arteries forward to the bifurcation is surrounded by thyroid follicles. Finally, thyroid 

 growth failing to keep pace with the lengthening of the ventral aorta, the adult con- 

 dition is approached when the thyroid mass breaks up into irregular deposits or clusters 

 of follicles which remain in the vicinity of the ventral aorta. 



In the adult trout the greater portion of the thyroid is dorsal to the aorta. Maurer's 

 description therefore requires that it migrate, from its origin dorsal to the heart tube, 

 to the ventral side of the aorta and then back again to the dorsal side, and this view is 

 confirmed by our specimens. 



While colloid no doubt appears very early in the embryo and before the latter 

 leaves the eggshell, Maurer's statement that it appears about the forty-first day of 

 incubation is indefinite, since incubation proceeds over a considerable range of tem- 

 perature and is much more rapid in the warmer water. The stage of incubation may 

 be accurately stated in temperature units when both time and temperature are known. 

 From the facts at hand, however, it may be expected that all normal trout embryos 

 have developed colloid as soon as the hatching stage is reached. 



In our specimens of hatchery brook trout still in the sac stage, but some days 

 hatched, colloid is plainly visible (fig. 13) in many follicles. The amount and location 

 of thyroid tissue is even at this early stage subject to considerable variation. The 

 follicles are, however, always relatively few. In three examples the total number of 

 distinct follicles recognizable by serial sections throughout the floor of the mouth was 

 24, 27, and 33, respectively. In other cases there appears to be a smaller number. In 

 distribution two more or less distinct deposits are recognizable, besides scattered folli- 

 cles. One of these is at or in front of the level of the epithelial invagination which is 

 to become the jugular pit, a region which will be shown later to be the seat of thyroid 

 deposits in the adult. At this stage it is already a very definite infolding. The follicles 



