DEVELOPMENT OF FLUSTRELLA HISPIDA. 457 



cells — that is to say, wliether it represents a process of in- 

 vagination, or whether the oral ectoderm simply grows over 

 and encloses the mesendoderm. In either case the process 

 may be regarded as a form of gastrulation, but no trace of an 

 archenteron has been observed. 



Later Segmentation Stages. — At the stage represented 

 in PI. 24*, fig. 60, except that the oral surface is slightly 

 flattened, the embryo has become almost spherical in form. 

 Aborally, it is covered by a layer of small, flat, ectoderm cells, 

 while the oral surface is composed of large, high cells; and 

 from these latter a mass of endoderm or mesendoderm cells 

 projects into the segmentation cavity, which is still visible at 

 this stage. A shallow groove, bounded below by a slight 

 protuberance, is noticeable on the exterior of the larva rather 

 above the equatorial line. A study of sections shows that 

 this groove marks the junction of the oral and aboral 

 ectoderm, and that it is due to the difference in the size of 

 the cells in this region. It is to be again noted that there 

 are no specially enlarged cells, such as have been described 

 by Barrois, in this region. 



Up to this stage in the development, the oral surface has 

 been relatively larger and more convex than the aboral, but 

 now cell division becomes more active on the latter surface, 

 so that it in turn becomes relatively larger and more convex 

 than the oral surface. Owing to the increased growth of the 

 aboral surface, the slight groove and the protuberance already 

 noticed now lie below the equatorial line, and in sections this 

 groove is now seen to be the result, partly of the difference 

 in size of the ectoderm cells at the line of junction of the oral 

 and aboral series, and partly of an actual bulging out of the 

 oral ectoderm (PI. 25, fig. 61, M.C.). This encircling groove 

 represents the initial stage of the aboral groove or mantle 

 cavity. 



The stage thus briefly described marks the close of the 

 segmentation period. The larva now passes gradually from 

 a spherical to an elongated form, which is typical of the 

 mature larva. When viewed in section (PI. 25, fig. 61), the 



