316 CHARLES EUGENE JOHNSON 



fourth pouch, and later manifests itself in the tendency to cyst 

 formation that is so characteristic of that body. Again, it 

 might conceivably be interpreted as a vestige of some other 

 derivative of the fourth pouch, such as a thymus. Opposed to 

 the first of these views, if not entirely to the second, is the fact 

 that in both embryos of Chelydra (15-mm. and 16-mm. c. 1.), 

 although on the right side only, an exactly similar vesicle occurs 

 in connection with the parathyreoid III (fig. 21). In the younger 

 specimen the cyst is largely surrounded by thymous tissue. In 

 the later series of Chrysemys the parathyreoid IV gives no evi- 

 dence of cyst formation, but* in an embryo of 15-mm. carapace 

 length parathyreoid III contains an excentric cavity of moderate 

 size whose wall is a single layer of cells, sharply differentiated 

 from the surrounding tissue. In an embryo at hatching there is 

 what appears to be a trace of such a cavity in the corresponding 

 gland; in the parathyreoid IV evidence of such condition is 

 doubtful. 



The ultimobranchial body in all of the later stages mentioned, 

 except that of hatching, shows merely a continuation of the proc- 

 ess of reduction of the vesicle, begun in some of the younger 

 embryos. In the present older specimens the vesicle is either 

 completely broken down into a mass of diminutive vesicles and 

 solid cell masses more or less spherical or cord-like in form, as in 

 Chelydra of 16-mm. carapace length (fig. 23) ; or the main cyst 

 is studded with sprouts and is extensively broken up and reduced 

 in size, as in an embryo Chrysemys of 11-mm. carapace 1-ength. 

 The process of reduction and transformation of the original 

 vesicle apparently takes place, chiefly, by two methods: by the 

 formation through evagination and separation (and perhaps 

 also simply by constriction) from the main body, of smaller 

 cysts of varying sizes and forms, and by the outgrowth and de- 

 tachment from its wall of solid cellular sprouts. In the sprouts 

 the cells at first have a radial or epithelial arrangement in section, 

 and while in some of them an actual lumen may appear, in 

 others such is seemingly not the case. The secondary vesicles 

 undergo further reduction in the same manner as the parent 

 structure. In the older specimen of Chelydra (16-mm. c. 1.) 



