722 J. F. GUDERNATSCH 



Maurer finds in the trout a primarily globular vesicle stretch- 

 ing in an antero-posterior direction, and on the 41st day of develop- 

 ment lying ventral to the stem of the aorta. If these statements 

 apply to all Teleosts, the thyreoid must first originate dorsal to 

 the aorta and early migrate ventrally and later return to a position 

 dorsal to the vessel, since it usually occurs there in the adult. The 

 condition in the Teleosts is similar to that in the Myxinoids, 

 where Stockard describes the origin of the thyreoid as a median 

 down-pushing from the ventral floor of the pharynx throughout 

 the entire gill area, and consisting, in newly hatched Bdello- 

 stoma, of diffusely scattered alveoli below the pharynx and 

 above the median branchial artery (ventral aorta). 



In the trout, where development is rather slow, Maurer observes 

 that 35 days after fertilization, when the embryo is about 6 mm. 

 long the first thyreoid vesicle begins to pinch away from the phar- 

 ynx. While originally the evagination, visible on the 28th day, 

 possesses a stratified epithelium, it has on the 35th day a single 

 layer of cuboidal cells. Three weeks later the whole stem of the 

 aorta is surrounded by follicles. I find in rainbow-trout, one 

 month old, or only 30 days older than those mentioned by Maurer, 

 that the majority of follicles, and the larger ones, lie above the 

 aorta, 



Maurer also observes that in the brook-trout shortly after the 

 first follicles have appeared the organ grows so rapidly that 

 for a considerable period it surrounds the aorta as a compact 

 mass. ''In very late embryos, the growth of the thyreoid does not 

 keep pace with that of the artery; thus the gland breaks away 

 from the aorta and separates into a number of irregular clusters of 

 different sizes lying either laterally partly paired or dorsal or 

 ventral to the aorta, always, however, in its immediate neigh- 

 borhood." He records the main mass of the gland in trout of 

 even 25 cm. as being compact and situated ventrally between the 

 second and third branchial arteries, and it is only in animals of 

 30-40 cm. that the thyreoid breaks up into the clusters of follicles 

 characteristic of the adult. Maurer describes the same conditions 

 in a number of other species, of which only the eel was at my 

 disposal. The age of the eel I examined was unknown, though 



