724 J. F. GUDERNATSCH 



Urodela a solid cylinder of epithelial cells from which the follicles 

 pinch away, exists for from two to four weeks. These cylinders are 

 observed in sheep and pig embryos up to the 20 cm. embryos and 

 in man up to the 24 cm. embryo. The follicles in fish are formed 

 comparatively earlier, and perhaps the gland functions earlier. 



Thus a rapid multiplication of follicles occurs in the Teleosts 

 without the formation of cell cylinders. This is an exception to 

 W. Miiller's claim that the thyreoid gland in all vertebrates passes 

 through three stages: (1) a severing of the anlage from the pharynx; 

 (2) formation of a network of tubes of glandular epithelium; 

 and (3) the formation of follicles from these tubes. In the Teleosts 

 and also in the Myxinoids, as Stockard has shown for Bdellos- 

 toma, the second stage seems to be suppressed or absent. 



The first appearance of colloid in the thyreoid gland is generally 

 thought to occur early in lower vertebrates but very late in the 

 higher ones, towards the end of fetal life or often not until extra- 

 uterine life. 



Maurer reports colloid in the trout thyreoid on the forty-first 

 day of embryonic development. How far this early appearance of 

 colloid is connected with the function of the organ is unknown. 

 From a comparative physiological standpoint it would seem that 

 in the lower forms the thyreoid might function much earlier than 

 in the Placentalia, where in intra-uterine life the gland of the 

 mother might supply the needs of the developing embryo.* 



HISTOLOGY OF THE GLAND 



The histological structure of the thyreoid gland in Teleosts has 

 been little studied. The meagre observations made by Baber in 

 1881, describing some features of the thyreoid in the conger eel 

 were the first reported. Maurer later ('86) mentions a few points 

 regarding the histology of the thyreoid in the trout and carp. 



The microscopic appearance of the gland varies as much as does 

 its anatomical structure. In sections from some specimens the 



' In young mammalian embryos Peremeschko found no colloid, in older embryos 

 it occasionally existed, while in young animals colloid was present in the majority 

 of follicles and in old ones in all of them. 



