208 J. H. ASHWORTH. 



by endodenn. 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 substance (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, Spz.), 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 continued 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 of a blunt, 

 conical, anterior piece fixed to the spherical nucleus. The length of 

 the head is 7 \x , the nuclear portion being 4 p, in diameter. The tail 

 is a slender filament about 27 fx long. 



The spermatozoa closely resemble those of Alcyonium in their 

 structure and development (Hickson, 1895). The endoderm covering 

 the sperm sac appears to undergo certain changes as the sperm sac 

 grows, and in thin sections from two to five nuclei may be counted in 

 many of the endoderm cells. One of these nuclei is sometimes larger 

 than the others. In the left upper cell of Fig. 35 the large nucleus 

 near the centre occupies the position of the original nucleus of the 

 cell. The other four nuclei have probably been produced from it by 

 division, but there has been no corresponding division of the protoplasm. 

 The cell with four nuclei which was figured by Hickson (1895, PI, 39, 

 Fig. 45, f.) from the teased preparations of the sperm sacs of Alcyonium, 

 was probably one of the cells of the endodermic folljcle. 



At first it appeared that the sperm sacs were situated on ventral and 

 lateral mesenteries only, although, as pointed out above (see p. 197), 

 the cells in the mesoglcea, from which the genital cells are derived, are 

 found in the, dorsal mesenteries as well. After examining a large 

 number of sections, I have found only two clear cases of sperm sacs 

 occurring on the dorsal mesenteries. In each case the sperm sac is 

 situated on the side of the mesentery a little distance from the free 



