A. L. COLWIN AND L. H. COLWIN 163 



Mechanism of Sperm Entry into Egg 



Little is known of the mechanism by which the movement of 

 the spermatozoon into the egg is effected once the acrosome fila- 

 ment has made its initial association with the egg. It is clear that 

 the mechanism is not direct contraction of the acrosome filament 

 per se, nor recession of the external part of the cone. Lillie's 

 (1912a and 1912b) description of sperm entry in Nereis offers 

 some indication of what the mechanism may be. As mentioned 

 previously, Lillie noted that the "perforatorium" had the form of 

 "the spike of a helmet" in living spermatozoa but that in fixed 

 spermatozoa, whether free or attached to an egg, the perfora- 

 torium was a straight filament-like structure. Metz and Morrill 

 (1955) have found that a comparable change in the acrosomal 

 region was induced by fertilizin treatment of Nereis spermatozoa. 

 It seems reasonable to consider, then, that Lillie's account of the 

 behavior of the spermatozoon and its perforatorium during sperm 

 entry is in reality an account of the behavior of the reacted sper- 

 matozoon and its acrosome filament during sperm entry. In Lil- 

 lie's description based on fixed material, the "fertilization cone" 

 becomes converted into a "specialized cell organ" identifiable by 

 its staining reaction. The sperm head becomes "anchored" in this 

 organ by the tip of its perforatorium (i.e., acrosome filament). 

 This complex, consisting of sperm head and cone, moves slowly 

 from the egg surface and penetrates more deeply into the egg. 

 About 64 minutes after insemination the sperm head becomes 

 separated from this cell organ. The further history of this "cone" 

 is unknown. 



Lillie's centrifuge experiments with inseminated eggs of Nereis 

 indicate that once the sperm head is associated with the sub- 

 stance of the cone the relationship is a firm one. There is some 

 evidence for such a finn relationship between the spermatozoon 

 and the substance of the cone in HolotJmria atra; in a few cases 

 it was found that if enough pressure were applied to the cover- 

 slip to cause the cone to rupture and form an exudate, the sper- 

 matozoon within the cone was not expelled ( Fig. 45 ) . Moreover, 

 in similar preparations in Thyone hriareus, the spermatozoon 



