156 



MORPHOLOGY OF FERTILIZATION 



readily distinguishable from the acrosome filament ( Fig. 49 ) . The 

 exceedingly long and tenuous acrosome filament spans the jelly 

 and reaches the egg surface. Presumably in response to a stim- 

 ulus from this filament, the egg forms a hyaline fertilization cone. 

 Figure 46 shows a scheme of the sperm entry process, and Fig. 



Fig. 46. a-f, scheme of sperm entry process in Thijone briareus. (From 

 Colwin and Colwin, Biol. Bull., 1956.) Part shown in dotted line represents 

 minimum additional length of acrosome filament presumed but not actually 

 seen to enter egg. g, reacted spermatozoon from alkaline sea water suspen- 

 sion for comparison of length of acrosome filament, a, presumed very early 

 stage, based on spermatozoon which failed subsequently to enter; b, low 

 broad incipient cone rises as acrosome filament proceeds into egg proper; 

 c-f, successive stages of sperm entry; acrosome filament enters egg proper 

 as most anterior part of spermatozoon (middle piece, left outside in this 

 case, frequently enters egg). In subsequent stages, entire flagellum passes 

 into egg proper. 



47 illustrates successive stages in one egg. Figure 48a-e and 

 Figs. 49 to 51 show stages in several other living eggs. The 

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

 the acrosome filament to varying heights and sunounds it like 

 a sleeve. In some eggs (Fig. 46) the cone rises around the fila- 



