162 MORPHOLOGY OF FERTILIZATION 



rising around a more delicate but already established acrosome 

 filament, as occurs in Thyone. In two of Chambers' (1930) cases 

 there is actually evidence that an acrosome filament remains in- 

 tact within the cone. In one case (his Fig. 10) the insemination 

 filament appeared to shorten progressively until the sperm head 

 finally reached the apex of the cone. But the head was rejected 

 and came to lie somewhat removed from the egg surface. At 

 that time a filament of some length extended from the tip of the 

 sperm head. In the second case (his Fig. 19) the egg had been 

 denuded of its jelly hull. No insemination filament was seen. 

 However, the sperm head which entered the cone was later ex- 

 pelled, and after expulsion a filament extended from the tip of 

 the head ( cf . the acrosome filament within the cone of Asterias in 

 Fig.48g-h). 



Horstadius described long narrow cones in the egg of the star- 

 fish Astropecten aranciacus ( 1939a) and of the holothurian Holo- 

 thuria poll (1939b). These cones were said to traverse the jelly 

 hull and "take possession of" the spermatozoon. His sketch of 

 the cone in this holothurian ( as well as in the starfish ) gives the 

 impression that a thin filament connects the sperm head to the 

 cone. It is now suggested that in both these cases what is por- 

 trayed is a slender cone which has risen around an acrosome fila- 

 ment already present as in Thyone (cf. Figs. 48a,e and Figs. 49- 

 51). 



Reinterpreted thus the descriptions by Fol, Chambers, and 

 Horstadius fall into harmony with the general picture of sperm- 

 egg association and sperm entry as they occur in Holothuria atra 

 and Thyone hriareus. The acrosome filament arises from the 

 sperm head and projects through the jelly to the egg surface; the 

 egg is stimulated and responds by cone formation. Depending 

 on the species and perhaps too on the degree of maturity of the 

 egg, the cone may vary from broad to very narrow, rise quickly 

 or slowly and to various heights, and may or may not form 

 advance projections. Whatever these variations may be, the cone 

 surrounds the already established acrosome filament like a sleeve, 

 and the filament (in some cases at least ) enters the egg proper 

 as an essentially intact structm'e. 



