No. 2.] THE ECHINODERM SPERMATOZOON. 257 



diminution in the diameter of this neck, together with a dimi- 

 nution in the size of the drop of cytoplasm at the tip, as well 

 also by a diminution of the cytoplasm which has hitherto re- 

 mained around the nucleus and the mitosome in the cell proper 

 (Figs. 35, 36, also Figs. 40, 42, 43, 44, 45, 50). The tail then 

 is the cytoplasm of the spermatid, which has become modified 

 in a very special way. It is possible that its violent motions 

 in the water are expressions of some molecular change which 

 the sea-water brings about in the protoplasm of the tail, for I 

 have often noticed that spermatozoa when first removed from 

 the testis into sea-water lie motionless ; after a short time a 

 slight motion begins, which after a few minutes increases to 

 the normal rapid motion. 



Pictet concluded that the tail is attached to the posterior 

 part of the nucleus, but certain facts seem to point otherwise, 

 for in those cases where the nuclei are caused to swell and 

 finally burst, the tail is almost invariably left attached to the 

 mitosome (Figs. 53, 54). Further in the process of fertiliza- 

 tion, the mitosome and the tail together separate from the 

 nucleus (Figs. 56, 57). On the other hand, however, is the ob- 

 servation made by Pictet and Cuenot, referred to above, that in 

 many cases, apparently with one species, Asthenosoma, as the 

 rule, the mitosome becomes detached from the nucleus before 

 the spermatozoon penetrates the tg%. It seems most probable 

 that the tail is in direct continuation with the cell membrane 

 which surrounds the spermatozoon. The cell membrane being 

 morphologically but the external slightly changed cytoplasm, 

 probably differs very little, morphologically not at all, from the 

 tail, and these two, the tail and the cell membrane, probably 

 pass insensibly into one another ; the mode of development 

 would seem to prove this. 



Cell memhranc. — A delicate cell membrane surrounds the 

 head of the spermatozoon, inclosing the nucleus, centrosome, 

 and mitosome. This membrane is best seen in cases where 

 from some mechanical cause a slight separation has taken place 

 between the nucleus and the mitosome ; this membrane in that 

 case being stretched but still unbroken (Fig. 55). It can also 

 be seen stretching over the centrosome in cases where the 



