PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 663 



In the reptiles, the gland is a single, lobate, follicular structure, 

 which contains colloid and lies close to the trachea, but in the 

 birds, it is paired, the halves lying close to the blood vessels and 

 the side of the neck (Verdun '98) large areas of which may be de- 

 void of colloid (Mrs. Thompson '10), and its vesicles are usually 

 closely compact. Whether the anlage is single or double in the 

 mammals, is not yet altogether settled, although Maurer ('02), 

 states that the evidence seems to be in favor of the single origin, 

 which would make it homologous with the gland in other 

 vertebrates. 



The parathyreoids, according to Verdun ('98), have their 

 homologues in the ' epithelial bodies ' of the amphibians, the latter 

 having first been described by Maurer ('88), and with this 

 identification I agree. (Concerning these structures more will 

 be said below.) 



Thymus. Whether the cyclostomes, like the gnathostomes, 

 have a true thymus is not settled. J. Miiller ('43), p. 115, was 

 the first to describe a paired structure situated behind the gills, 

 in the region of the heart in the myxinoids, which he, at that time, 

 called 'Nebennieren.' Later, in an editor's note to Stannius' 

 ('50) work on the thymus gland of the fishes (p. 507) he suggests 

 that the ' Nebenieren' previously described by him corresponds to 

 the thymus of the fishes, and thus it was that Stannius ('54) in 

 his Handbuch (p. 256) says, '^Die paarige Thymus is bischer bei 

 den Myxinoiden .... beobachtet worden," and in a foot- 

 note on the same page he remarks, ' 'Bei den Myxinoiden ist dies 

 gebilde aufgefunden und beschrieben von J. Miiller, der es zuerst 

 als Nebennieren, spater als Thymus deutete." The statement 

 of Beard ('94, p. 485), that "the only positive statement we 

 possess emanates from Stannius," is not therefore exact, although 

 his interpretation of this organ as a degenerate pronephros in 

 Myxine, may be correct. Stockard ('06), p. 99, in his study of the 

 thyreoid in Bdellostoma, found no evidence of a thymus gland in 

 any of the stages of development, and doubts its existence. 



On the other hand, Schaffer ('94) described the development 

 of seven small bodies on the dorsal as well as the ventral region of 

 as many pharyngeal pouches in Petromyzon, which he interprets 



