A. L. COLWIN AND L. H. COLWIN 145 



eggs of the same species (Figs. 1-13 and cf. Figs. 1 and 2 with 

 Figs. 24 and 25). In all species the reaction resulted in the com- 

 plete disappearance of the original acrosome and the production 

 of a slender rigid filament. The length of the filament varied with 

 the species, being in each case about three times the length of 

 the sperm head. The longest filament measured about 15 microns. 

 In the reacted spermatozoa the middle piece spherules appear to 

 have become less tightly affixed to the base of the sperm head. 

 In some species the reacted condition was observed also in 

 "supernumerary" spermatozoa, that is, in spermatozoa which 

 were associated with the egg (Figs. 5 and 8) but did not sub- 

 sequently enter it. In some species spermatozoa were found in 

 which the acrosome had undergone only a "partial reaction" 

 (Figs. 3 and 6), supposedly because of subnormal condition of 

 the spermatozoa or suboptimal stimulation. In these spermatozoa 

 the filament was always about the length of the original unreacted 

 acrosome. 



Rothschild and Tyler ( 1955 ) have reported that fixed sperma- 

 tozoa of the chiton Lepidochitorui cinerea, seen with the electron 

 microscope, have an "acrosomal filament" or "anterior process" 

 about twice as long as the rest of the head, whereas in living 

 spermatozoa, seen with the light microscope, "the proximal por- 

 tion of the process is the only part visible." Until a comparison is 

 made between fixed and living spermatozoa under one kind of 

 microscope, a different interpretation cannot be excluded, namely, 

 that the two different relative lengths of the process might repre- 

 sent reacted and unreacted conditions in fixed and living sperma- 

 tozoa, respectively. Some factor in the initial phase of fixation 

 might have stimulated the acrosome reaction. 



Dan's important observation that spermatozoa undergo an 

 acrosome reaction in response to various stimuli has been con- 

 Fig. 23. Electron micrograph of a reacted (egg water treated) sperma- 

 tozoon of Asterias amtirensis; black arrow points to acrosome filament. 

 (From Dan, Biol Bull, 1954.) 



Figs. 24-25. Electron micrographs of formalin-fixed spermatozoa of 

 Mytilus edtilis. (From Dan and Wada, Biol Bull, 1955.) 24, in sea water 

 (control). 25, reacted (egg water treated); black arrow points to acrosome 

 filament. 



