438 



thyroid region cau be detected. In specimens in which a rudiment 

 of a tail 3 mm in length persists the basihyo-branchial muscle has 

 undergone little or no degeneration, and at its anterior border can 

 be found numerous capillaries corresponding in their connections to 

 the vascular network of the later appearing pseudothyroid. As this 

 vascularized area lies anterior to the basihyo-branchial muscle and to 

 the attachment of the anterior part of the sternohyoid muscle to the 

 processus branchialis, the possibility is precluded that at this 

 time a "Kiemen rest" has migrated to this region from the branchial 

 chamber. The changes that occur between the stages represented 

 in specimens of 13 mm and of 17 mm length are as follows: The 

 degenerating muscle disappears, although sometimes portions of it 

 persist until the pseudothyroid developes a lymphoid structure (Fig. 7). 

 The pseudothyroid lymph recess makes its appearance and com- 

 monly communicates with the tortuous passages representing the 

 sinus Sternalis in this region (Fig. 5). But it is not always easy 

 to distinguish between lymph passages and empty blood vessels. 

 Between the external jugular vein and the pseudothyroid lymph recess 

 is the vascularized area above mentioned, whose vessels communicate 

 with the vein in several places. Round lymphoid cells make their 

 appearance in the meshes of the network of the vascularized area, the 

 latter increases in size and projects into the lymph recess (Figs. 5 

 and 6). 



After the disappearance of the cavity of the branchial chamber 

 (in frogs of about 14 mm length) the branchial remains consist of 

 three not very sharply separated portions : a dorsal part connected with 

 the floor of the mouth cavity and, as in later stages, projecting into 

 the parahyoid lymph sinus, a middle portion, and a small ventral 

 part lying near a recess of the spatium sternohyoideum. This 

 small ventral portion disappears early, while the other two remnants 

 persist for some time. This ventral portion, however, never has any 

 connection with any possible anläge of the pseudothyroid. Before this 

 ventral remnant disappears the anläge of the pseudothyroid exists as 

 a distinct structure situated some distance from it. 



In frogs of 17 mm length the Corpus prop er i car dial e in 

 the form of an unpaired transversely elongate lymphoid mass lies in 

 the sinus stern alls anterior to the pericardium, ventral to the 

 hypoglossal muscles and a little posterior to the place of their 

 union (Fig. 1). The structure of the C. propericardiale at this 

 stage is similar to that of the pseudothyroid, — . a mass of oval- 

 elongate lymphoid cells infiltrated with a network of capillaries. 



