THE STKUCTUliE OF XENIA HIOKSONI. 275 



that the gonads are, in this as in other Alcyonaria, endo- 

 dermic in origin. 



Each sperm sac originates as a slight projection at the side 

 or free edge (except in the dorsal mesenteries) of the mesen- 

 tery, 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 

 by endoderm. When the sperm sac has reached a diameter of 

 about -06 mm. to -08 mm. there appears in the centre a small 

 cavity, free from nuclei, but containing some coagulable sub- 

 stance (fig. 31, Cg.). This central cavity continues to enlarge 

 for some time, along with the growth of the sperm sac (fig. 32), 

 and attains its maximum size in sacs about "2 mm. to '25 mm. 

 in diameter, in which the central cavity reaches a diameter 

 about one fourth that of the sperm sac. After reaching this 

 size it is gradually encroached upon by the heads of the 

 ripening spermatozoa (fig. 33, /S^j^.),and in the fully developed 

 sperm sac, which is about '35 mm. in diameter, the cavity has 

 completely disappeared, the whole of the interior of the sperm 

 sac being filled with a mass of somewhat loosely packed 

 spermatozoa, many hundreds in number, produced by the con- 

 tinued division of the single primitive genital cell. There are 

 no examples of karyokinesis in the many hundreds of sperm 

 sacs I have examined. 



The head of each ripe spermatozoon (fig. 34) consists 



