718 J. F. GUDERNATSCH 



ture below the aorta and carries the blood directly into the sinus 

 venosus. 



Little is known about the relation of the thyreoid gland to the 

 lymph system. This is largeh t due to the fact that in the fishes 

 the lymph vessels are in a much closer connection with the venous 

 system than in the higher vertebrates. It is almost impossible 

 to distinguish between veins and lymphatics bj^ the injection 

 method. 



In many species large cavities lie around the aorta, two dorsal 

 ones being constant in trout. These are extraordinarily large and 

 lined with endothelium and although they often contain blood 

 corpuscles there is little doubt that they are lymph sinuses. The 

 corpuscles probably come in from the venous system, or possibly 

 by traumatic haemorrhages. A further fact in favor of their 

 being lymph sinuses is that no descriptions of large veins in this 

 region has come from the numerous injections of the circulatory 

 system of Teleosts. In some species there is only one large 'lymph 

 sinus' which surroujids the aorta dorsally and laterally. 



DEVELOPMENT OF THE THYREOID 



Only one contribution deals with the embryology of the thy- 

 reoid in Teleosts, this is by Maurer ('86) who traces its develop- 

 ment in the brcok trout. The gland arises in much the same 

 manner as it does in the other classes of vertebrates. 



The thyreoid develops very early in the Teleosts, after the 

 first gill slit has broken through, ^ as an unpaired evagination of 

 the stratified epithelium on the ventral side of the pharynx between 

 the first and second gill pockets. It is thus placed in the curve of the 

 S-like tubular heart before the gill arteries have developed, with the 

 exception of that to the hyoid arch. The vesicular thyreoid anlage 

 very soon separates itself from the pharynx and enlarges by bud- 

 ding. The organ lies close to the tubular heart, but only remains 

 for a short time near the place of its origin. With the development 



^ In Hertwig's Handbuch d. Entwicklungslehre II, 1, 1906, Maurer states, how- 

 ever, that the anlage of the thyreoid appears in all Gnathostoma before the break- 

 ing through of the first gill slits. 



