PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 617 



darkly staining substance, and the large epithelial cells have 

 divided into numerous smaller ones. 



The third body (about 100 micra caudad to the ear) is almost 

 spherical, about 40 micra in diameter, and is slightly flattened 

 dorso-ventrally. It has moved slightly dorsally and laterally, so 

 that it lies dorsal and lateral to the cartilage of the first branchial 

 arch, and immediately dorsal to its levator arcuum branchiarum 

 muscle. 



The fourth body is close behind the third (about 40 micra) , but 

 is a little dorsal and medial to it, directly above the second bran- 

 chial cartilage and lateral to its levator arcuum branchiarum 

 muscle. Like the third it is almost spherical, varying in diameter 

 between 37 and 43 micra. 



The fifth body is some distance (about 110 micra) caudad to the 

 other two, and is more dorsal than either, lying close to and a 

 little above the level of the middle region of the brain. It is oval 

 and measures 44 x 53 micra. 



E. Amblystoma larvae, 19 or 20 mm. long. Since the thymus 

 bodies take positions which remain practically constant for all 

 succeeding larval stages and their morphological relationships are 

 well defined, the description is given in some detail. As a basis 

 for this I use a wax model of a 19 mm. larva. 



The three bodies have increased somewhat in size, but retain, 

 on the whole, their general spherical or slightly oval form. The 

 third and fourth are close to each other (about 95 micra apart), 

 the fifth, however, is quite a distance caudad to these two (nearly 

 285 micra). The fact that the fifth body is relatively farther 

 caudad to the fourth, than the fourth is from the third is noted 

 both by Maurer ('88, Axolotl) and Maximow ('12, Siredon); 

 neither however give the exact distances. 



A dorsal view of the reconstructed parts, showing the three thy- 

 mus bodies on the right side of the body is shown in figure 41. In 

 size, the third and fourth bodies are about equal, being 90 x 75 

 micra and 75 x 60 micra respectively, while the fifth is somevvhat 

 larger than either, measuring 90 x 130 micra. It is interesting 

 to note here that the fifth body in Amblystoma at this stage of 

 development is nearly the same size as that given for the fifth 



