156 ALICE M. BORING. 



the cells thus taking on a triangular rather than a rectangular 

 shape in section. This is shown in Fig. D. Some sections, how- 

 ever, do not show this at all, and we must conclude, that in some 

 cases the endoderm lags a little behind the ectoderm. 



During the subsequent process of closure, the ectoderm and 

 the endoderm stretch across from both sides till they meet and 

 form a two-layered membrane which closes in the digestive tract 

 again. In this process of closing the two layers keep nearly 

 even, but the ectoderm sometimes is a little farther advanced 

 than the endoderm and curves around over its free end. The 

 closing membrane does not arch outwards so as to complete the 

 cylindrical form, nor does it grow straight across, which would 

 be the shortest distance between the sides, but it is nearly always 

 concave (Figs. C and D). In pieces which are much less than a 

 half-cylinder, there is little chance for the membrane to bend in, 

 and it lies close to the old wall (Fig. E). Also in cases where 

 there is a large endodermal ridge, the bending in is either pre- 

 vented or lessened. 



The position of the endodermal ridges and their relation to 

 the closure of the stem must now be considered. Godlewski 

 has not taken these ridges into account, although one or two of 

 his figures seem to indicate their presence. These ridges are 

 found throughout the stem and cannot be ignored. Their pres- 

 ence causes some variations in the beginning of the process of 

 closing and introduces certain conditions which might be easily 

 misinterpreted. If the cut extends along the side of a ridge, or 

 through it, the appearance in section of such a piece is very dif- 

 ferent from a section in which the ridge is not near the cut edge. 

 There is a thick mass of cells at the edge, appearing somewhat 

 like the beginning of a closing membrane (Fig. F). This mass 

 is chiefly endoderm cells which contain many yellow-stained 

 granules. If only a little of the ridge has been cut off, it is easily 

 identified as a ridge by the thickening of ectoderm outside of 

 it containing numerous dark stained nuclei ; otherwise, identi- 

 fication is possible by tracing back through sections to a place 

 where the cut was less deep and the ridge still intact. The ecto- 

 derm stretches over the cut endoderm until the two layers are 

 even and then the two stretch across together and meet to form 



