PHARYNGEAL DERIVATIVES OF AMBLYSTOMA ^ 619 



(d.L), muscle of the side of the neck. The laterahs nerve of the 

 vagus passes caudad just medial to it. 



All three bodies are similar in their histological appearance. 

 They are loosely imbedded in the connective tissue, the matrix 

 of which is composed of fine fibers. Within the bodies, the mar- 

 gins of which are not clearly defined, are the larger epithelial 

 cells, and, in addition to these, numerous other cellular elements 

 which conspicuously modify the general appearance of the bodies. 

 Nearly all of the yolk has disappeared, and the cells uniformly 

 take a darker stain. There is no membrana propria, the larger 

 marginal cells being apparently held in place by a loose reticulum, 

 which penetrates not only the spaces between the outer cells, but 

 also extends inward throughout the spaces in the central area. 

 The size and shapes of the epithelial cells differ in different sec- 

 tions. (For further' detailed description of the histogenesis of 

 the thymus bodies of this stage, refer to Maximow '12, 19 mm. 

 Siredon larva, p. 597) . 



F. Ambly stoma larvae, 25 to 1^5 mm. long. As noted above, the 

 position of the thymus bodies in the succeeding larval stages up 

 to metamorphosis (46 mm.), changes but little, so that only a 

 brief consideration need be given them here. 



There is little change in size and position of the bodies in a 26 

 mm. larva, from that just described. The third, on the right side, 

 is almost spherical, about 90 micra in diameter, and lies just dor- 

 sal to the first efferent branchial artery; the fourth is slightly oval 

 (87 X 75 micra); and the fifth (110 x 130 micra), Ues some con- 

 siderable distance caudad. No great difference in the histological 

 appearance is noted. In section (fig. 29) the fifth body (which 

 is typical of the others also) has an outer layer of rather large 

 epithelial cells, which enclose a central cavity of some size. 

 There is no membrana propria, the cells being separated one from 

 another by strands of connective tissue. 



A wax reconstruction was made of the three bodies in the 35 

 mm. larval stage of development, but this shows little difference 

 from that of the 19 mm. stage. The bodies have increased in size ; 

 the fourth is larger than the third, and the fifth is the largest of 

 all. The third is oval (102 x 93 micra); the fourth is the same 



