BRANCHIAL DERIVATIVES IN TURTLES 313 



able, but, especially in Trionyx, on the side where the vesicle is 

 rudimentary, the fourth pouch derivative may at times assume 

 a very similar form, so that the two may be distinguished with 

 certainty chiefly by their relative position. 



The following account is based upon an embryo Chrysemys of 

 11-mm. carapace length, one of 15-mm. carapace length, and one 

 at hatching; two embryos of Trionyx of carapace length of 9 

 mm. and 13 mm., respectively, and two of Chelydra of carapace 

 length of 15 mm. and 16 mm., respectively. 



The thymus and parathyreoid bodies are now readily distin- 

 guishable from each other, both as to structure and staining 

 properties. The thymus has taken on the characteristic lym- 

 phoid appearance and stains deeply. The parathyreoid, on the 

 other hand, exhibits its usual cord-like, epithelial cell masses, 

 with invasions among them of mesenchymal tissue; these features 

 together with the relatively greater amount of cytoplasm in the 

 cells and their less deeply staining nuclei contrast this organ 

 sharply with the thymus. In regard to the relation of the thymus 

 to the carotid artery, Chrysemys and Chelydra are in accord and 

 differ from Trion\Ti. In the former two the artery is situated 

 laterally, having changed from an earlier, more ventral position. 

 In Trionyx the vessel courses along the medial surface of the 

 gland, but in an earlier stage it was near the ventral surface. In 

 both groups, if a large series were examined, a considerable 

 amount of variation would no doubt be found in the degree of 

 rotation of the thymus about the artery. The parathyreoids are 

 apparently also quite variable, within certain hmits, as to their 

 position in the later stages. In Trionyx, where they are some- 

 what less advanced than in the other two forms, the organ of the 

 left side lies on the ventromedial, while that of the right lies on 

 the ventrolateral surface of the thymus, slightly anterior to its 

 caudal end. In Chrysemys the parathyreoid III is on the medial 

 side of the thymus, more or less deeply imbedded and separated 

 from the carotid by a considerable mass of thymous tissue; 

 in Chelydra its situation is lateral or dorsolateral upon the thymus 

 adjacent to the carotid in the younger specimen of this genus, 

 but in the older it is found to have been shifted somewhat and 



