302 CHARLES EUGENE JOHNSON 



as the more favorable stages are at hand to illustrate the suc- 

 cessive steps in the developmental processes. For the later stages 

 a number of specimens of Trionyx also are available. As in 

 lizards, the first gill pouch does not give rise to any parts of the 

 organs under consideration and may therefore be omitted from 

 further reference in this connection. 



In an embryo Chrysemys of 4 mm. the first three visceral 

 pouches are clearly differentiated. Just behind the third pouch 

 is a fourth conspicuous evagination from the lateral pharyngeal 

 wall. In form this diverticulum is more rounded than the pre- 

 ceding pouches. Its lateral wall is flattened and has rather 

 broad contact with the ectodermal epithelium; and on its dorsal, 

 ventral, and posterior walls constrictions occur in the sections 

 by which the limits between the diverticulum and the pharyn- 

 geal wall proper appear clearly defined, but anteriorly these 

 limits may be recognized only in a general way. The size and 

 form of this evagination readily distinguish it from the typical 

 visceral pouch and in its walls appear no differentiations that 

 might indicate, as such, the developing fourth and fifth visceral 

 pouches or the ultimobranchial diverticulum. In this specimen 

 the first pair of the associated aortic arches is complete, but the 

 second and third pairs are visible in their dorsal portions only. 



A somewhat more advanced condition is shown in a 5-mm. 

 Chelydra (fig. 1). The first three pouches have increased in 

 depth. The first and second are already perforate and the third 

 is nearly so. The second aortic arches are conspicuous, but 

 reach only about half way to the ventral aorta. The third arches 

 are now complete, as are also a very slender pair of fourth arches. 

 The diverticulum behind the third pouch has grown very con- 

 siderably and two distinct areas or divisions in its wall are 

 now discernible: an anterior larger part which is elongated in the 

 dorsoventral direction, parallel with the third visceral pouch, 

 and a second smaller part which appears as a diverticulum from 

 the first, pushing out from its posterior wall. The former is in 

 close contact with the ectoderm nearly throughout its length; 

 and on its inner surface, along this line of contact, it presents 

 a conspicuous furrow. In the hght of subsequent stages, the 



