600 F. L. LANDACEE AND A. C. CONGER 



Stage XIV. Stage XIV, 6 hours older (fig. 27) than the pre- 

 ceding, shows no striking changes, in the auditory vesicle and 

 preauditory placode, which are still attached to the ectoderm 

 throughout most of their extent, although the posterior end of 

 the vesicle is detached through a slightly greater extent than in 

 the preceding series. The most evident changes are in the ecto- 

 dermal thickening connected with the gill pocket. The detail of 

 this thickening is shown in figure 7 which is taken anterior to 

 that of figure 27, and lies just behind the area of contact of the 

 first true gill. 



This thickening has extended somewhat further backward and 

 now reaches nearer to the posterior end of the auditory vesicle 

 which here, as in the preceding series, lies nearer to the medulla 

 oblongata than to the ectoderm. The posterior portion of the 

 thickening extending behind the area of contact is in this series 

 somewhat longer dorso-ventrally, so that it extends down from 

 its position in the preceding series to the angle (fig. 27) where 

 the lateral body wall joins the nearly horizontal wall which ex- 

 tends over the yolk sac. In a later stage the first gill pocket 

 thickening will separate from a second one associated with the 

 second true gill and the first will be found to end blindly in the 

 ectoderm as in the case of the hyoid gill thickening. There is, 

 however, only a slight indication of a division into two in this 

 series. 



Stage XV. In this stage the posterior half of the auditor}^ ves- 

 icle is detached from the ectoderm. The pharyngeal pocket of 

 the second true gill has not reached the ectoderm. When it 

 does reach the ectoderm its anterior end would lie directly under 

 the area of contact of the preceding gill, and the same condition 

 is true of the remaining gills, so that their ultimate arrangement 

 is like the shingles on a roof, the anterior end of each endodermic 

 pharyngeal pocket lying ventral to the preceding area of contact. 

 Posterior to the area of contact in each pharyngeal pocket there 

 is the thickening of the ectoderm as in the case of the hyoid gill. 



While the pharyngeal pocket of the second true gill has not 

 reached the ectoderm, the ectodermic thickening over the area of 

 future contact is present and slightly separated by a groove on 



