376 SYDNEY J. HICKSON. 



A few round interstitial cells may be seen in the endoderm 

 in nearly all good sections through this region. These cells 

 are of different sizes, and probably replace and supplement 

 the other endoderm-cells during the animal's life. 



The endoderm covering the mesenteries is composed of 

 short cubical cells which become flattened and fusiform over 

 the muscular ridges on the ventral faces of the mesenteries ; 

 but the general appearance of this part of the endoderm, as well 

 as that just described in the last paragraph, varies very much 

 with the condition of contraction or expansion of the polyps. 

 When the polyps are fully expanded, it is a thin flat layer of 

 cells; but when they are contracted the cells are long, thin, and 

 so crowded together that, in sections which do not pass through 

 it quite vertically, it has the appearance of consisting of two 

 or three layers. 



The sheath of endoderm which covers the gonads and the 

 endoderm forming the six mesenterial filaments are not affected 

 in this way by the condition of the polyps. In the former case 

 the cells are cubical in shape, finely granular in consistency, 

 without verv distinct walls, and without muscular processes 

 (fig. 25). ' 



The sections of partially retracted tentacles that I have made 

 show that the endoderm in that condition is a compact tissue 

 composed of short cubical cells with round nuclei. I can find 

 no muscular processes connected with any of the cells, and I 

 am convinced, after a prolonged study of these cells, that they 

 are not present. Each cell is very finely granular. There are 

 no nematocysts. The lumen of the tentacles is wide, and it is 

 continued for a short distance into each of the pinnae (fig. 10). 

 The endoderm of the pinnae is similar to that lining the axial 

 cavity of the tentacles. 



The sexual cells first make their appearance in the thickened 

 border of the ventral mesenterial filaments. In both sexes 

 they occur as a mass of polygonal cells (fig. 37) covered by a 

 single layered sheath of endoderm. There can be no doubt 

 whatever of their endodermic origin. 



Oogenesis. — If a series of sections be made through a 



