436 



branchial chamber requires altogether too great a readjustment of 

 adjacent structures. Moreover, the facts of development gainsay any 

 such supposition. 



After the metamorphosis, in young frogs (R. fusca), of about 

 17 mm length the pseudothyroid is seen as a lymphoid knot pro- 

 jecting into a lymph space, lateral and somewhat posterior to the 

 thyroid gland, and ventral to the processus postero-lateralis 

 of the hyoid cartilage (Fig. 1). The external jugular vein runs along 

 its lateral border, and the greater part of the body lies anterior to 

 the junction of the latter vein with the pharyngeal vein. The lymph 

 space mentioned is a recess of the sinus lymphaticus sternal is, 

 although at this stage of development it is not uniformly in com- 

 munication with the latter. A communication is sometimes found be- 

 tween the pseudothyroid lymph recess and the sinus parahyoideus. 

 At this time the only vestiges of the gills still persisting are two 

 rudiments on each side, one dorsal to the carotid gland and the 

 second postero-ventral to the aortic arch. The first is a remnant of 

 the dorsal portion of the degenerating gills, while the second comes 

 from a middle region of the gill-remains. 



Passing from this stage of development when the pseudothyroid 

 first assumes a characteristic lymphoid structure, to a stage shortly 

 after the tail of the tadpole has disappeared, in frogs of 13 mm 

 length, there is seen in the position exactly corresponding to the 

 situation of the pseudothyroid as above described a vascular network, 

 in the meshes of which are degenerating muscle fibres (Figs. 2, 3 and 

 4). Examination of still earlier stages of development shows that 



CO prth 



dm 



Fig. 2. Cross-section througli the head of a youug frog (R. . fuscaj of 13 mm 

 length; parts dorsal to the pharj-nx removed. In the median mass (cp) ventral to the 

 hyoglossus muscle blood vessels are seen developing, extending laterally to the region 

 of the external jugular vein, be branchial chamber with remains of degenerated gills 

 etc.; bhb M. basihyo-branchialis, degenerating fibers; ca A. carotica; cp denser reti- 

 culum, anläge of corpus propericardiale ; cjyJiy cornu prineipale of hyoid cartilage; mm 

 fragments of degenerated branchial muscles; sc sinus ceratohyoideus. Other parts as 

 in Fig. 1. 



