314 CHARLES EUGENE JOHNSON 



has become partly imbedded in the thymus (figs. 21, 22). Re- 

 garding the growth changes in the parathyreoid in these later 

 stages my series is too small to furnish definite answer, but from 

 measurements in Chrysemys it seems that, while the thymus 

 increases greatly, the parathyreoid III suffers a cessation or 

 retardation of growth in size between the stage of 15-mm. or 

 16-mm. carapace length and that of hatching. 



In the embryo Chrysemys of 11-mm. carapace length the fourth 

 pouch derivative shows structural and staining characteristics 

 identical with those of the parathyreoid III. It lies somewhat 

 isolated from the derivatives of the third pouch and I find no 

 evidence of thymus tissue in connection with it on either side 

 of the body. The derivative of the fourth pouch, therefore, 

 at least from the evidence in this case, is a parathyreoid body 

 only, but it is quite possible that a thymus sometimes is developed 

 also. In the present specimen the parathyreoid IV has suffered 

 little if any change in position from that of this derivative of 

 earher stages, being situated upon the dorsolateral surface and 

 slightly caudal to the anterior end of the ultimobranchial body; 

 lateral to it appears the posterior tip of the thymus III. The 

 parathyreoid IV of the right side, which is somewhat larger than 

 its fellow, still has the rudimentary ultimobranchial body at- 

 tached to its ventral surface. As stated in connection with the 

 9-mm. Trionyx, the fourth pouch derivative was inclined to be 

 more vesicular than in the other two genera during the early 

 stages, and the same tendency appears in the older embryos 

 now concerned. In the smaller of these (9-mm. c. 1.) it has an 

 appearance not unlike that of the ultimobranchial vesicle, but is 

 smaller. The body of the right side especially is large and thin- 

 walled (fig. 17) and caudally has developed three secondary 

 out-pouchings from the main vesicle, giving to the whole still 

 more the character of an ultimobranchial body. In the older 

 embryo (13-mm. c. 1.) the bladder form is even more pronounced, 

 but here this feature may involve only a part of the entire organ. 

 Thus, on the left side, the fourth pouch derivative consists of a 

 ventromedial solid mass and a dorsolateral bladder-Uke portion 

 in which the wall is extremely thin and apparently in process of 



