439 



The blood vessels at this time have essentially the same arraügement 

 as described by Gaupp for the adult condition. From each side 

 comes a small artery given off from the ramus musculo-glandu- 

 laris of the external carotid artery, the same branch that supplies 

 the pseudothyroid. On each side there passes laterally a vein, a 

 continuation of the Vena pseudothyreoidea. As noticed by 

 Gaupp the arrangement of the blood vessels is subject to a great 

 deal of variation. In earlier stages this region of the sinus sterna- 

 lis, in which the C. prop e ricar dial e lies, is occupied by loose 

 reticular connective tissue. In frogs of 13 mm length no sinus 

 stern alis is found at the level corresponding to the position of the 

 C. propericardiale (Fig. 2). The reticulum in the median region 

 is somewhat denser than in other portions. An examination of earlier 

 stages shows that this denser part where the lymphoid structure will 

 later appear corresponds to the median ends of the two basihyo- 

 branchial muscles of the tadpole. In frogs of 13 mm length blood 

 vessels have already made their appearance in the reticulum above 



ps rps 

 \ 







^^/^'fe^,/' 



,H 



/ «'A«: 



-fA2 



hy vcp sthy 



Fig. 5. Cross-section through the right pseudothyroid of a frog (R. fuscaj at a 

 stage slightly older than that shown in Figs. 2, 3 and 4. Tlie pseudothyroid is just 

 beginning to take on a lymphoid character, mdi V. maudibularis interna; ps pseudo- 

 thyroid; ssl irreguLir lymph spaces that later form the anterior portion of the sinus 

 sternalis ; th- remnants of a median thyroid gland; vcp parts of the? vein that later 

 passes from the corpus jjropcricardialc to the external jugular vein ; vhj anläge of the 

 vein that later ])asses from the hyoglossus muscles into the corpus propericardiale. 

 Other parts as in preceding figures. 



