4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 52 



one during the feeding season, unless, possibly the chromaphil tissue 

 in the former may be somewhat more in evidence than in the latter. 

 The tissue available, however, was not sufficiently abundant to deter- 

 mine this point with entire certainty. 



THE THYROID AND PARATHYROID GLANDS 



The thyroid gland in the alligator is a small, bilobed structure 

 (pi. I, fig. 4, ty) lying across the ventral surface of the trachea at 

 the level of the auricular end of the heart. In a 40 cm. animal, the 

 gland is of the size shown in figure 4. It consists of an oval or 

 spherical lobe on each 'side and an interlobular portion or isthmus. 

 The isthmus, is relatively thick and short so that the lateral lobes are 

 not sharply distinguishable from it as in some of the higher verte- 

 brates. The gland has a reddish color and is easily removable without 

 distortion from the body. The microscopic structure of the gland 

 does not present any very unusual features. It is surrounded by a 

 fibrous capsule (pi. i, fig. 5, c), which seems to consist of two layers ; 

 the capsule has a tendency to separate, as shown in figure 5, into an 

 inner and an outer part. 



The alveoli are, in some cases, very closely arranged ; in other 

 glands they are more widely separated by interstitial connective 

 tissue, blood vessels and empty sinuses. In some cases the majority 

 of the alveoli contain colloid ; in other cases most of them are empty. 

 Higher magnification shows the alveoli to be lined with the usual 

 single layer of cubical or columnar cells (fig. 6) with large, spherical 

 or compressed nuclei. The cells are clear or finely granular, the 

 nuclei are coarsely granular. Even in tissue that seems otherwise 

 perfectly fixed the walls between the cells can seldom be demon- 

 strated. In some alveoli the cells are large ; in other alveoli, especially 

 in those free of colloid (pi. i, fig. 6, A) the cells are small and seem 

 compressed, so that adjacent nuclei are almost in contact with each 

 other. 



Closely adherent to the lobe of the thyroid gland (fig. 5, pt) is a 

 very small body which, at first, looks like scarcely more than a 

 thickening of the fibrous capsule of the thyroid. More careful ex- 

 amination, however, brings to light certain structures that would 

 seem to indicate that this small body is either the parathyroid or the 

 post-branchial body or probably both. Swale Vincent (11) says that 

 these two bodies are closely united in some of the other reptiles and 

 that they are subject to much variation. He figures the combined 

 bodies from some unnamed animal but from the figure it is difficult 



