484 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



At the time of the appearance of the five radial excisions soon after the rupture 

 of the vestibule the pavement epithelium of the inner side of the five flaps con- 

 taining the orals can always be demonstrated as such ; but gradually it transforms 

 itself into a cuticlelike membrane without nuclei. Whether the nuclei become 

 simply absorbed, or whether they retreat to the deeper layers of the body wall, 

 has not been determined. The latter, however, is more probable, since before a 

 homogeneous cuticlelike membrane is formed there is on many places a complete 

 fusion of the ectodermal pavement epithelium with the mesenchyme, whereby the 

 nuclei of the ectoderm cells, with the extremely attenuated plasma surrounding 

 them, become united with a process from a mesenchyme cell. On the other hand, 

 the inner investment of the orals in the oldest larva? of this stage sometimes shows 

 a pavement epithelium passing over uninterruptedly into a membrane devoid of 

 nuclei, which must have arisen from the transformation of such an epithelium. 



Whether in still older larvae the inner side of the orals is covered exclusively 

 by a membrane Seeliger could not determine from lack of proper material; but 

 it is not improbable that the transformation of the original pavement epithelium 

 is carried through to its logical conclusion. 



Inwardly the pavement epithelium passes over into the outer wall of the 

 tentacles, and between these into the floor of the vestibule, where an entirely 

 different histological structure has now arisen. Outwardly, on the border of the 

 oral flaps, it goes over directly into the cuticle formed from the ectoderm of the 

 outer body wall. 



The basal wall of the vestibule, which extends out between the tentacle bases 

 and leads into the esophagus, decreases considerably in thickness toward the 

 outer border of the calical cavity, at the same time here spreading itself over a 

 broader surface. On account of this the nuclei, previously lying in several layers, 

 become rearranged in such a way that they now lie approximately in a single 

 layer. This condition is most marked about the borders, so that the hydroccele 

 ring is now covered on its inner side with a very fine pavement epithelium, which, 

 at first running vertically, bends about the aboral border of the hydrocrele ring 

 into the horizontal oral disk, simultaneously gradually altering its histological 

 nature. 



The component cells are outwardly cubical, becoming in the vicinity of the 

 mouth high prisms bearing on their outer ends cilia which appear embedded in 

 a clear cuticular border. 



Immediately about the mouth the epithelium resembles that of the esophagus, 

 into which it passes over without a definite line of demarcation. According to 

 the state of contraction this transition zone is sometimes drawn into the funnel- 

 shaped invagination as an esophageal wall, and at other times spread out 

 horizontally as circumoral disk epithelium. Thus, in contrast to the aboral side 

 of the larva, the oral side preserves the epithelial character of the ectoderm. 



It is the epithelium of this transition zone, designated by Barrois as the 

 intratentacular area, or secondary vestibule, which later spreads out over the 

 entire ventral side of the disk and also throughout the ambulacral region of the 



