A. L. COLWIN AND L. H. COLWIN 151 



expressly for the filamentous structure or projection which occurs 

 in the acrosomal region when the spermatozoon undergoes the 

 acrosome reaction and that the term be avoided when referring 

 to the unreacted acrosomal region whatever its length may be. 



Some Physical Properties of the Acrosome Filament 



Shape. The threadlike acrosome filament is generally rather 

 straight although this is not invariably the case. The filament 

 may curve gently, or sharply, or even bend ( cf . various examples 

 in Figs. 26-35). Although it appears to have considerable rigidity, 

 it is nevertheless somewhat flexible. This was shown by Dan for 

 certain starfishes (Fig. 21), and it has been found so for other 

 starfishes and also for Thijone (Fig. 27). In some cases the acro- 

 some filament was observed to break. Occasionally spermatozoa 

 were seen in which it appeared that the head had reacted but 

 the acrosome filament was much shorter than usual, or simply not 

 present. Possibly breakage may account for these exceptional 

 specimens. 



Length. Dan and Wada (1955) have indicated that there 

 does not seem to be a simple taxonomic relationship between 

 species and length of acrosome filament. However, it is evident 

 that a positive correlation exists between the length of the fila- 

 ment and the depth of the barrier through which the sperma- 

 tozoon must pass in order to meet the egg proper. For example, 

 in a number of sea urchins the jelly hull offers little or no barrier 

 and the spermatozoon readily swims through it to the egg; in 

 these the acrosome filament is very short. In contrast, the egg 

 jellies of various starfishes and holothurians ofl^er effective bar- 

 Figs. 28-35. Unretouched photographs of living preparations. (From 

 Colwin and Colwin, Biol Bull, 1956.) Black arrow points to acrosome fila- 

 ment, white arrow to flagellum. The scale is in microns and applies only 

 to Figs. 28-34. 28-31, spermatozoa of Thijone hriareus: 28, unreacted sper- 

 matozoon; 29-30, long acrosome fikiments on reacted spermatozoa in alka- 

 line sea water; 31, egg water treated spermatozoon with acrosome filament; 

 32, reacted spermatozoon with acrosome filament in contact with egg 

 surface ( no cone formed ) . 33-34, reacted spermatozoa of Asterias forbesii 

 showing acrosome filament in contact with the egg surface (no cone 

 formed). 35, reacted spermatozoon of Asterias amurensis showing the 

 acrosome filament in contact with the egg surface (no cone formed). 



