A. L. COLWIN AND L. H. COLWIN 153 



the surface. Subsequently, this suggestion has received strong 

 support from observations on sperm entry in several other species 

 (Colwin and Colwin, 1955a, 1956). From the above considera- 

 tions it seems likely that the acrosome filament of a species would 

 have to be at least as long as the depth of the barrier which sep- 

 arates the approaching sperm head from the reactive egg surface 

 in that species. Within a given preparation (e.g., in Thyorie) 

 spermatozoa may be found with acrosome filaments of different 

 lengths. Some filaments are shorter than the depth of the barrier 

 around the species egg. It has been observed frequently that the 

 first spermatozoon, or even the first several spermatozoa, to reach 

 a ripe egg may not succeed in fertilizing it. Inadequate length of 

 the filament might account for such failure. 



Sperm-Egg Association and Sperm Entry 



The acrosome filament now may be examined in relation to the 

 egg during sperm entry. The following descriptions are based 

 exclusively on living material. 



Holothuria atra. The exceptional clarity of the entrance cone 

 in this species made it possible to observe many details readily 

 (Colwin and Colwin, 1955a). The eggs were removed from the 

 gonads. Most were in the germinal vesicle stage when used. Al- 

 though all were moderately polyspermic and failed to cleave, 

 they were considered (op. cit.) to be close to physiological ma- 

 turity and to reflect the essentials of the normal entry pattern of 

 the species. 



The egg is surrounded by a jelly hull ( Fig. 37 ) through which 

 the spermatozoon does not swim. The chief events of sperm entry 

 are shown diagrammatically in Fig. 36, and representative stages 

 are shown in photographs in Figs. 39 to 44. At the earliest obser- 

 vation an incipient cone was already present, and the acrosome 

 filament extended to the cone through the jelly hull. The main 

 points to be noted are as follows. ( 1 ) The cone rises around the 

 acrosome filament. Often one projection of the cone creeps up 

 the filament in advance of the main body of the cone (Figs. 39- 

 40). For a time at least, the apex of the cone moves outward 

 while the spermatozoon moves inward. Indeed, the cone may 



