224 THE BIOLOGY OF THE FROG chap. 



there has been much discussion, but of whose function 

 nothing positive is known. 



It is probable that blood corpuscles are produced to a 

 certain extent in the thymus (Mayer). According to Abe- 

 lous and Billard, if both thymus glands of the frog are 

 removed, the animal soon dies, after a period of great mus.- 

 cular weakness, ulceration of the skin, and a variety of other 

 pathological symptoms. Hammar, however, failed to con- 

 firm these results. He found that both thymus glands may 

 be removed from the frog without injury, and concludes that 

 the results obtained by Abelous and Billard were the effects 

 of accidental infection. 



The Pseudothyroid and the Epithelial Bodies. — The 

 pseudothyroid and the cpitJiclial bodies are organs of similar 

 structure and origin. They are derived from the modifica- 

 tion of the epithelium of the gill slits of the larva and are 

 therefore products of the entoderm. The two p^udo- 

 thyroids are the largest of these. They are rounded 

 reddish bodies, lying on either side of the posterior portion 

 of the hyoid cartilage. They were formerly mistaken 

 for the thyroids, but they possess a very different inter- 

 nal structure, which is essentially that of a lymphoid 

 gland. 



The epithelial bodies are small, rounded structures usually 

 more than two in number on each side and somewhat vari- 

 able in position, but generally situated near the pseudothy- 

 roids. As an organ probably belonging in the same cate- 

 gory as the preceding may be mentioned the propericardial 

 body, which is a transverse oval organ lying ventral to the 

 hyoglossus muscle between the thyroids. It possesses a 

 lymphoid structure and is larger in young than in old frogs 

 (Gaupp). From its mode of development Maurer classes 

 the carotid gland also among the epithelial bodies, but its 



