A. L. COLWIN AND L. H. COLWIN 149 



about 60 microns, and the depth of the jelly hull, through which 

 the acrosome filament must penetrate in order to reach the egg 

 surface, is approximately 55 microns. The spermatozoa of Thyone 

 do not swim through this jelly hull. 



Other Species. In Holothuria atra (Colwin and Colwin, 

 1955a), acrosome filaments were found on spermatozoa sus- 

 pended in egg water (with egg albumin as an adjuvant) and 

 on spermatozoa associated with the egg surface (Fig. 38). In 

 starfishes (Colwin and Colwin, 1955a,d, 1956) the acrosome fila- 

 ment has been found on spermatozoa in association with the egg 

 in Asterias amurensis (Fig. 35), A. vulgaris (Fig. 27m) and 

 A. forbesii (Figs. 27k,l and 33 and 34). In all four species sper- 

 matozoa with acrosome filaments were found also in the surround- 

 ing medium but not directly associated with any egg. There is 

 also evidence of acrosome filament formation in spermatozoa of 

 the annelid Sabellaiia vulgaris (Colwin and Colwin, 1955b). 



Metz and Morrill (1955) have demonstrated that fertilizin 

 treatment of spermatozoa of Asterias forbesii and the annelid 

 Nereis liinbata results in the production of acrosome filaments. 

 In Nereis the acrosome membrane breaks down and a rod-like 

 filament is exposed (or arises); the overall length of the sperm 

 head remains unchanged. This material was fixed in formalin 

 and examined with the electron microscope. 



From the foregoing evidence it is quite clear that in many spe- 

 cies, representing several phyla, the spermatozoa undergo rather 

 profound changes when suitably stimulated. The most striking 

 change is the production of the more or less straight, relatively 

 rigid acrosome filament. In some species the reaction has been 

 elicited by egg water, in some by alkaline sea water or by surface 

 "contact," but in others it has been seen thus far only when the 

 spermatozoa are in the vicinity of, or in contact with, unfertilized 

 species eggs (some molluscs). As is well known, a number of 

 stimuli other than the normal one (that is, association with the 

 opposite gamete) may cause parthenogenetic activation of the 

 egg. It is not surprising, then, that the spermatozoon too may be 

 activated by other than normal stimuli. 



It is suggested that the term "acrosome filament" be reserved 



