THE STRUCTURE OF XEXIA HICKSONI. 207 



taper at their ends into long slender processes which become connected 

 with similar processes of adjacent cells. 



Some of the cords of cells ?re, however, obviously formed by 

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

 endoderm. 



Spermatogenesis (PI. XXII., 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 

 ccelentera of several of the older polyps (PI. XX., 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 pentagonal 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. 197), have migrated to their present position in the 

 mesogloea from the endoderm, so that the gonads are, in this as in 

 other Alcyonaria, endodermic in origin. 



Each sperm sac originates as a slight projection at the side or free 

 edge (except in the dorsal mesenteries) of the mesentery, and consists 

 of one of these genital cells covered by a thin sheet of mesogloea, and 

 by a single layer of endoderm cells continuous with the endoderm of 

 the sides of the mesentery. The genital cell, the protoplasm 

 of which is finely granular, or sometimes contains small vacuoles, is 

 spherical and about "01 mm. in diameter, and in the centre has a large 

 spherical nucleus whose diameter is about half that of the cell. 



The nucleus and protoplasm of the genital cell undergo division, 

 which at first is apparently regular, as many cases of four or eight 

 cells so produced may be seen (Fig. 30). The divisions of the genital 

 cell are accompanied by divisions of the endoderm cells covering it, and 

 very soon the increase in size of the sperm sac causes it to project as a 

 spherical or oval body from the mesentery, to which it always remains 

 attached by a stalk consisting of a thin cord of mesogloea surrounded 



