Vol. 3] Torrey. — Biological Studies on Corymorpha. 275 



numerous than elsewhere. The causes suggested for the changes 

 in the endoderm of the tentacle anlage will apply equally well 

 here. Fig. 195 is a median section showing the beginning of 

 the evagination. The subsequent course of events involves, first, 

 the emigration of several cells in a single row, which push out 

 the ectoderm and thin it before them. 



The resemblance of a frustule at this stage (Figs. 20, 21) to 

 a proximal tentacle (Fig. 16) is remarkably striking, especially 

 when it is remembered that the fate of the two as well as their 

 origin in the history of the race are so different. Similar me- 

 chanical problems appear to be meeting with similar mechanical 

 solutions. 



The frustule soon turns downward and, running close along 

 the column, gradually lengthens. Its tip becomes clavate, ter- 

 minating a more and more attenuated stalk. Just how many 

 cells enter it from the enteric epithelium it is impossible to say. 

 ]\iitotic figures are again absent, and though the rod of f rustular 

 endoderm becomes exceedingly narrow and delicate at its base, a 

 direct connection with the endoderm of the column can be dem- 

 onstrated at a stage much later than that in which the tentacular 

 endoderm is separated from parent epithelium by the mesogloea 

 A definite delimiting layer of mesogloea is probably never devel- 

 oped, though it is also quite probable that the endodermic epi- 

 thelium does not contribute cells throughout the period during 

 which the frustule elongates. It is certain, as will be seen below, 

 that the elongation of the frustule is not wholly dependent upon 

 an increase in number of axial cells, from whatever source they 

 may come. 



In the adult the first visible change noted in the development 

 of a frustule is an increased granulation of the ectoderm cells 

 in a small circular area over a peripheral canal. The granules 

 stain conspicuously with both haematoxylin and orange G. At 

 the same time, as can be seen in sections of the stem, the granu- 

 lar cytoplasm of the endoderm cells under this area stains more 

 deeply wuth haematoxylin, a change evidently correlated with 

 a simultaneous thinning of the mesogloea from the inner surface. 

 The ectoderm cells take on a higher, narrower, columnar form. 



