622 FRANCIS M. BALDWIN 



and the trapezius muscles and the skin of the lateral surface of the 

 body. It is about 240 micra long (its long axis parallel to the long 

 axis of the body) , but is flattened laterally, its shortest axis is less 

 than 100 micra. 



The fourth, similar in shape and position, is close behind the 

 third (only one or two sections intervene, about 60 micra), but it 

 is a little dorsal to the latter and is flattened against the dorso- 

 laryngeus muscle. It is about 270 micra long and 60 micra in 

 minor diameter, and somewhat narrowed at its cephalic end. 



The fifth thymus body is close behind the fourth (about 60 

 micra) , and is flattened laterally and closely pressed against the 

 muscles of the region. It is the largest of the three, discus- 

 shaped, measuring 500 micra in its major, and about 75 micra 

 in its minor diameters. 



H. The thyinus gland of the adult. Since the thymus gland in 

 certain adult Amblystomae has been the subject of several de- 

 scriptions, from both the morphological and histological stand- 

 points (Simon, '45, Axolotl; Leydig, '52; Siebold and Stannius, 

 '56; Fischer, '64, Siredon; Bolau, '99, A. tigrinum; Dustin, '11, 

 Axolotl and others) the description here is brief. In addition 

 to two series of microscopical slides through the head and neck 

 region of A. punctatum (one just metamorphosed, the other an 

 older adult), several specimens (A. tigrinum, 15 cm. and 22 cm. 

 long; A jeffersonianum, etc.) were dissected. Although minor 

 differences occur in the location and appearance of the gland in 

 the individuals, on the whole its relationships are fairly constant. 

 Variability in the gland was noted by Bolau ('99) who says, 

 ' 'Ein Constanz im Auftreten der Thymus der Zahl nach habe ich 

 bei Amblystoma tigrinum selber oder bei der weissen Varietat 

 nicht finden konnen. Die Zahl der Driisen wechselte nicht nur 

 individuell, sondern ich fand auch Verschiedenheiten bei dem- 

 selben Thierauf den beiden Seiten." The following description 

 is based upon the dissection of A. tigrinum. 



The gland is exposed (fig. 37) by removing the skin from the 

 side of the neck caudal and dorsal to the angle of the jaw. It is 

 generally a light gray, somewhat elongate, flattened, three-lobed 



