616 FEANCIS M. BALDWIN 



The fifth is some distance caudad (about 120 micra) to the fourth 

 and is slightly oval in form, extending through three successive 

 sections, having a minor diameter of 20 micra. In both fourth 

 and fifth bodies, the epithelial cells retain the light yolk laden 

 character of the cells of the pharynx and there is no well defined 

 outer membrane. 



D. Amblystoma larvae, 13 to 15 mm. long. Except for the per- 

 sistence of the anlage of the first body on the right side in one 

 larva 13 mm. long, both the first and second thymus bodies on 

 either side have now degenerated. The persisting anlage is re- 

 duced to a short finger-like stalk of cells extending dorsally from 

 the pharynx at a point between the medial ventral surface of the 

 quadrate cartilage and the facial ganglion, just under the anterior 

 wall of the ear. Its epithelial cells are small and thinly scattered, 

 and the light yolky character has disappeared. In the 15 mm. 

 larva, the degeneration of the first two bodies is complete, no 

 trace of them being found in subsequent stages. This early 

 degeneration of the first two bodies in Amblystoma, agrees in the 

 essential points with facts recorded by Maurer ('88), in Axolotl, 

 and Driiner ('04) in Siredon, and Maximow ('12) in Siredon. 

 Driiner, however, found an epithelial follicle in two of his later 

 stages (36 mm.), which he interpreted as remains of the first thy- 

 mus body. He also found a similar follicle in an older larva (the 

 exact length of which he does not state). Careful examination 

 of the region of the first thymus body in Amblystoma in subse- 

 quent stages has thus far failed to show such a follicle as Druner 

 describes. 



Shiftings of the surrounding parts, due to growth, are taking 

 place more slowly now, so that the third, fourth and fifth thymus 

 bodies are relatively little changed in position. All are round or 

 slightly oval in form, and vary slightly in size from each other and 

 from their fellows on the opposite side of the body. The margins 

 of the bodies, as in the earlier stages, are not well defined, the cells 

 being imbedded in an intercellular matrix, there being no mem- 

 brana propria. The contained yolk, so conspicuous in the early 

 stages is now rapidly disappearing, being replaced by a more 



