A. L. COLWIN AND L. H. COLWIN 161 



tozoon even though part of the jelly hull appeared physically to 

 separate the spermatozoon from the egg. He concluded that the 

 egg must send an "attraction cone" to meet the spermatozoon. 

 However, before reaching this conclusion Fol speculated that 

 some connecting filament might arise from the sperm head and 

 extend to the egg surface. He discarded this view because he 

 could not see such a filament. It is now clear that the hypothesis 

 rejected by Fol for A. glacialis is satisfied in Holothuria atra and 

 Thyone briareiis by the demonstration that the spermatozoon 

 does make contact with the egg surface by means of the acrosome 

 filament and that the cone subsequently rises around the fila- 

 ment. It is therefore suggested that what Fol ( 1877 ) described 

 as a fine thread of protoplasm growing from the cone to establish 

 communication with the spermatozoon was, in fact, a projection 

 of the cone growing up around the acrosome filament which had 

 already established the initial communication. 



Chambers (1923, 1930), working with Asterias (forbesii?), 

 adopted essentially the view of Fol. According to Chambers, a 

 long straight "insemination filament" grew out from the cone to 

 make contact with the sperm head and then contracted, dragging 

 the sperm head through the jelly toward the cone. In fresh mature 

 eggs, however, no cone was seen which was not already con- 

 nected with the sperm head by means of a filament. It is now 

 suggested that the filament described by Chambers in the fresh 

 eggs was the acrosome filament and that the insemination fila- 

 ment which appeared to grow out to meet the sperm head in 

 other cases was, in fact, a very slender projection of the cone 



Figs. 49-54. 49-51, unretouched photographs of sperm entry as seen in 

 living specimens of Thyone hriareiis. (From Colwin and Colwin, Biol. Bull., 

 1956.) (Figures have had nonrelevant pieces spliced into upper portions.) 

 Scale of Figs. 28-34 applies. Cones of slender type. Note acrosome filament 

 within cone. 49—50, successive views of one specimen. 52-54, vmretouched 

 photographs of sperm entry in living material of Saccoglossus kowalevskii. 

 (From Colwin and Colwin, /. MorpJiol. 1954b.) 52, a polyspermia egg 

 showing a filament extending from sperm head to and into a very early cone 

 (the heads of two nonentering spermatozoa lie outside the egg membrane). 

 53, a normal egg with a filament between sperm head and early cone; a 

 narrow projection of the .cone extends up the filament. 54, a normal egg with 

 filament from sperm head apparently extending into hyaline portion of cone. 



