140 MORPHOLOGY OF FERTILIZATION 



the "acrosome reaction" and spermatozoa which showed the 

 changed appearance were called "reacted" spennatozoa. In most 

 species, egg water and/or the presence of unfertilized eggs were 

 effective stimuli. Although these conditions might be supposed 

 to approach those encountered at fertilization, it was found also 

 that the changes could be evoked in other ways, such as by 

 treatment with alkaline sea water, or by "contact" with solid 

 surfaces (e.g., glass slides, collodion membranes). It was not 

 demonstrated whether the causal agent was contact per se or 

 some other as yet undetermined factors associated with the han- 

 dling of the preparations. Dan's studies included two species of 

 sea urchin {Pseudocentrotus depressus and Strongylocentrotus 

 pulcherrimus) , three species of starfish {Asterina pectinifera, 

 Asterias amurensis, and Astropecten scoparius), and twelve spe- 

 cies of bivalve mollusc. 



Sea Urchins. In studies of living spermatozoa by phase con- 

 trast microscopy and of whole formalin-fixed spermatozoa by 

 electron microscopy, Dan (1952) found that suspension in egg 

 water would induce the acrosome reaction. The reaction was in- 

 terpreted as a breakdown of the acrosomal membrane so that 

 the contained acrosomal material was exposed. Part of the ex- 

 posed material was dispersed rapidly. There remained a central 

 core or fiber which Dan ( 1954 ) termed the "acrosome filament" 

 (Fig. 16; cf. with the untreated spermatozoon, Fig. 18). The 

 length of the filament was about one micron, short as compared 

 with the length of the sperm head. A similar reaction occurred 

 when sea urchin sperm suspensions were placed in alkaline sea 

 water of pH 9.2. Frequently the reacted spermatozoon (Fig. 17) 

 showed the middle piece displaced to a lateral position while the 

 flagellum appeared to emerge from the head at an angle instead 

 of lying in the long axis of the body. Reacted spermatozoa with 

 acrosome filaments were found also in untreated control suspen- 

 sions. Dan interpreted these to be the result of "contact" stim- 

 ulation with a solid surface. 



Rothschild and Tyler ( 1955 ) studied sea water sperm suspen- 

 sions from two other species of sea urchin. In Echinocardium 

 cordatum they found two forms of the spermatozoon and con- 



