THYKEOID GLAND OF THE TELEOSTS 719 



and shifting of the heart and aorta as well as by its own growth 

 the thyreoid gland comes to lie far from its original position. The 

 absence of a capsule of connective tissue similar to that in higher 

 vertebrates admits the loosening and separation of the thyreoid 

 follicles in the bony-fish.^ 



The Teleosts show a condition of the thyreoid gland somewhat 

 similar to that in M3rxine glutinosa, as W. Miiller, Cole, 

 Schaeffer and Maurer state. The follicles in Myxine, partly isolated, 

 partly in groups, are found betw^een the phary^nx and the truncus 

 arteriosus throughout the gill region. In the Teleosts, how- 

 ever, the gland also extends below the truncus. In the skate 

 Baber observes "sl single body and a few detached vesicles"; 

 in the Amphibians separated particles have also been described. 

 Yet in both of these groups the thyreoid possesses a capsule, which 

 sends septa into the inner portions, as W. Miiller has shown for 

 Acanthias and Raja. Maurer finds a delicate connective tissue 

 capsule in the Urodela. These observations on Selachii and Am- 

 phibia, however, are exceptional and the small detached particles 

 can only be looked upon as 'aberrant thyreoids' the main thyreoid 

 in all cases being a sharply defined body. Maurer observed in the 

 Urodela that a breaking up of the thyreoid into smaller parts 

 occasionally occurred. These accessory thyreoid glands were 

 parts of the former isthmus which, after the anlage had divided, 

 persisted and remained in their original position, while the true 

 halves moved in a postero-ventral direction. In the Ophidia the 

 organ is compact and encapsuled; but in the Saurii, according to 

 W. Miiller, the interstitial tissue increases so much through the 

 accumulation of fat, that the glandular tissue proper is broken 

 up into irregular groups which are sometimes completely discon- 

 nected. We have here a dissolution within the capsule suggesting 

 that the connective tissue capsule is the only factor in other verte- 

 brates which prevents the thyreoid elements from becoming 

 scattered about as they are in Myxinoids and Teleosts. 



* The elements of the Teleost pancreas are similarly scattered in the mesen- 

 terium. 



