CHESTER. — STRUCTURE OF PSEUDOPLEXAURA CRASSA. 753 



(1) the iutor-stitiiil cells (c/. ///.), or some of tliein; (2) the loosely ;ir- 

 rungecl cells (rl. grn' .) with few i;niiuiles, and those with larger grunules; 

 (3) the cells on the outside of the cords, which are either partially or 

 wholly surrounded by secreted matter; and (4) the smaller mesogloea 

 cells (c/. ms'gl.); because the interstitial cells and the series of loosely 

 arranged cord cells are continuous with eacli other. The cells of each 

 series may change their shape by amoeboid movements, as sectioned 

 li\ing nuiterial has shown me. Any of the cells, except the meso- 

 gloeal cells, may have granules. The cytoplasm and the nuclei of all 

 these cells are alike, except in regard to the size and shape of the 

 cell and nucleus and the presence of granules. Bat it would not be 

 correct to argue from this series of cells found in the coenosarc that 

 the mesogloea is of exclusively ectodermal origin. The bases of the 

 ectoderm cells seem active in the formation of mesogloea in all parts 

 of the colony and there is evidence of the secretion of the same sub- 

 stance by cells associated with the axis epithelium in the axis region. 

 The e\idence drawn from Pseudoplcxaura does not exclude the 

 probability that the endoderm is also active in the formation of 

 mesogloea in the tentacle, polyp wall, and particularly in the mesen- 

 teries. The small cells at the end of my series of four given above 

 occur in most abundance where the jelly layer of the coenenchyma is 

 most dense; they simply represent, it seems to me, the ultimate con- 

 dition of cells whose usefulness may not be limited to the formation 

 of mesogloea, but which in all of the earlier stages have been more or 

 less active in the formation of such substance. 



But not all of the mesogloeal cells belong in this series, nor are all 

 those in the cords secreting cells. Some of the interstitial cells develop 

 nematocysts and some are spicule-forming cells. Those forming 

 nematocj'sts are very abundant in the coenosarc, and not only are 

 stages in the development of the cyst found, but cysts {nmcys.) 

 as large and as fully formed as any near the surface are very abundant, 

 not only among the interstitial cells near the ectoderm, but also in the 

 cell cords and in the deep parts of the coenenchyma. These, I 

 believe, have been carried in from the ectoderm by the rapid growth 

 of the outer layer and by the amoeboid action of the cells around them. 



In growing regions at the tip of the branches some of the interstitial 

 cells, pushed deeper by growth, secrete the spicules, probably during 

 special periods. The formation of the spicules is not different from 

 that described by von Koch ('87), Bourne ('99), or Woodland (:05). 

 The more or less rounded spiculc-cell first shows a small calcareous 

 mass (Plate 3, Fig. 23, spc), which increases in size with the di\isioa 



