A. L. COLWIN AND L. H. COLWIN 165 



lized eggs elicit this reaction in spermatozoa of one or more 

 of the species studied. In some species the reaction is also 

 elicited by "contact" with a surface and, presumably, other factors 

 as yet unknown. Both quantitative and descriptive data are 

 presented for the acrosome reaction as observed in living sperma- 

 tozoa of Thyone briareus. 



2. The length of the acrosome filament varies from species to 

 species, ranging from about 1 micron in some sea urchins to 75 

 microns or more in Thyone in which it sometimes exceeds the 

 length of the flagellum. It is suggested that in any species the 

 length of the filament of a successful spermatozoon can never be 

 less than the depth of the barrier which effectively separates the 

 sperm head from the reactive egg protoplasm. The jelly hull 

 acts as a barrier in the starfishes and holothurians studied but not 

 in the sea urchins. 



3. At the very earliest stage of sperm-egg association observed 

 in Holothuria atra and Thyone, an acrosome filament is already 

 present and extends through the jelly hull to an incipient cone. 

 The cone rises around the acrosome filament to varying heights, 

 surrounding all or part of the filament in a sleeve of protoplasm. 

 In some cases the sleeve is so narrow as to make this portion of 

 the cone itself seem like a filament. The acrosome filament enters 

 the egg intact as the most anterior part of the spenn head and 

 remains essentially unaltered in diameter and length until well 

 into the egg proper, at least in H. atra and probably also in 

 Thyone. Partial evidence suggests essentially the same course 

 of events in several starfishes and an enteropneust. 



4. Some of the earlier descriptions of sperm entry in echino- 

 derms are reinterpreted as follows. The filament seen between 

 sperm head and cone (Fol, Chambers) is not of egg origin but 

 is the acrosome filament of the spermatozoon; the narrow cone 

 which appears to rise from the egg to meet the spermatozoon 

 (Horstadius) is simply rising around the acrosome filament 

 which has aheady established contact with the egg proper. 



5. Little is known of the mechanism which eftects the move- 

 ment of the spermatozoon into the egg, but this mechanism is not 

 direct contraction of the acrosome filament nor recession of the 

 external part of the cone. 



