274 J. H. ASHWORTH. 



mesogloea in intimate communication with the coelentera^ the 

 canals, and the external ectoderm. Bourne (1895) has shown 

 that in Xenia umbellata these rings of cells are ectodermic 

 .on account of their connection with, and general resemblance 

 to, the ectoderm of the free part of the poly|), and the occur- 

 rence of spicules and nematocysts in some of them. 



The cords of cells in the mesoglcea are then chiefly ecto- 

 dermic, as they arise from the cylinders of cells around the 

 coelenteron, or, in the outer portion of the mesogloea, migrate 

 inwards from the inner irregular surface of the external ecto- 

 derm. The cells all present a similar appearance, being either 

 rounded or rather elongated in shape, with somewhat vacuo- 

 lated protoplasm. The elongated cells frequently taper at 

 their ends into long slender processes which become connected 

 with similar processes of adjacent cells. 



Some of the cords of cells are, however, obviously formed 

 by obliteration of the lumen of a small canal ; these cells are, 

 of course, endoderm. 



Spermatogenesis (PI. 27, figs. 30 — 35). 



The specimen is a male, and shows beautifully all the stages 

 in the development of the spermatozoa. 



Gonads are most numerous in the upper portion of the stems 

 of the colony, but many sperm sacs are found in the basal 

 part of the free portion of the polyps, and a few also in the 

 lower portions of the colony. Sections through the upper 

 portion of the stems show that sperm sacs are so numerous 

 that they practically fill up the cavity of the coelentera of 

 several of the older polyps (PL 35, fig. 8, S. S.). In these 

 cases most of the sperm sacs are no longer spherical, but by 

 mutual pressure have become angular, being usually penta- 

 gonal or hexagonal in section. 



The genital cells are derived from the cells which lie in the 

 mesogloea of the mesenteries near their inner or free edge. 

 These cells, as shown above (see p. 2G3), have migrated to 

 their present position in the mesogloea from the endoderm, so 



