276 



University of California Puhlications. [Zoology 



The endoderm cells become smaller and more numerous. Fig. 

 22 shows the beginning of evagination of both endoderm arid 

 ectoderm. The layer of endodermic muscle fibres is interrupted 

 where the epithelial cells push outward in the shape of a solid 

 rod. At the same point, the mesogloea thins out and arches out- 

 vrard, accompanied by the ectoderm. The latter soon dimin- 

 ishes perceptibly in thickness with the shortening and broaden- 

 ing of its columnar cells (Fig. 23). The endoderm meanwhile 

 arranges itself in the single row of cells characteristic of the 

 well developed frustule, and pushes through 

 the original mesogloea. These points are 

 shown in Figs. 23 and 24; the latter is very 

 slightly oblique but does not distort the 

 facts, at the same time exhibiting the deri- 

 vation of frustular axial cells from enteric 

 epithelium. 



The frustule next develops a thick, slightly 

 bulbous base and a narrowed extremity, its 

 axial cells continuing to show conspicuously 

 the green granules so characteristic of the 

 epithelium from which they came. In the 

 course of further development the base nar- 

 rows until, in a fully developed filament, it is 

 almost impossible to distinguish where it joins 

 the stem. Finally, the filament degenerates, 

 having created a perisarcal anchor for the stem. 

 Fig. 25 shows that the extent of the granu- 

 lar ectoderm, w'hich is the first indication of 

 the frustule from a surface view, increases up 

 to the time when the ectoderm of the frustule 

 begins to thin at the tip, and then gradually 

 decreases as the ectoderm of the frustule thins out progressively 

 toward the base. In all probability this means that many ecto- 

 derm cells are being transferred from column to frustule, there 

 to be modified in form in accordance with the lateral tension 

 under which they appear to be placed by the elongation of the 

 axial rod. 



Fig. 22. Early 

 stage in develop- 

 ment of frustule 

 on adult stem. 

 X 360. 



