A. L. COLWIN AND L. H. COLWIN 159 



afro and there is at least partial entiy in T. hriareus. There is 

 collateral evidence for this picture of sperm entry in the follow- 

 ing species as well. 



Astericis. In A. amurensis (Colwin and Colwin, 1955a) the 

 acrosome filament was seen within the cone. In A. forbesii (Col- 

 win and Colwin, 1956) the filament was sometimes seen to be 

 within the cone and to extend a short distance into the egg proper 

 (Figs. 48g-h). Rarely it was observed to be well within the egg 

 proper, as in Thy one (cf. Fig. 48f ). 



Saccoglossus kowalevskii. In this enteropneust, too, similar 

 events appear to occur during sperm-egg association and sperm 

 entry (Colwin and Colwin, 1954b). The egg is not surrounded 

 by a jelly hull. Instead there are two membranes, an outer and an 

 inner one, Membranes I and II, respectively (Colwin and Col- 

 win, 1954a). From the earliest observation spermatozoon and 

 egg are connected by a filament. As the cone develops it surrounds 

 a portion of the filament. In some cases an advance projection 

 from the broader base of the cone creeps up the filament. This 

 was observed both in polyspermic and in normal eggs (Figs. 52- 

 54). 



After Dan's original (1952) publication on the acrosome reac- 

 tion it was "conjectured that in the egg of S. kowalevskii a similar 

 acrosomal breakdown, occurring when the spermatozoon reaches 

 the outside of Membrane I, might discharge a threadlike projec- 

 tion tluough the outer and the vitelline [II] membranes and that 

 this structure or substance, on reaching the true egg surface, 

 might elicit the reaction of the fertilization cone" (Colwin and 

 Colwin, 1954b). The observations on Holothiiria atra and Thijone 

 hriareus, as well as the collateral evidence from other species, 

 tend to support this view that cone formation is a response by 

 the egg to stimulation by the acrosome fialment. This view was 

 expressed also by Dan ( 1954 ) . 



A Reinterpretation of Earlier Descriptions 

 of Sperm Entry in Echinoderms 



Fol ( 1878-79 ) noted that in Asterias glacialis a slender tapering 

 cone arose from the egg surface and grew toward the sperma- 



