158 MORPHOLOGY OF FERTILIZATION 



cases the acrosome filament was seen to extend into the egg 

 proper for some distance (Figs. 46-48). In a few other cases the 

 filament could not be identified as such, but a long narrow dis- 

 turbance in the arrangement of the yolk granules extended to a 

 depth approximating that to which the filament could be pre- 

 sumed to have entered, as judged by the position of the head. 



Fig. 48. Variations in sperm entry, from sketches of living specimens. 

 (From Colwin and Colwin, Biol. Bull, 1956.) a-f, Thijone briareiis; g-h, 

 Aster ias forbesii. Acrosome filaments shown only to depth actually seen, 

 a-b and c-d, successive views of two specimens, respectively; e, very slen- 

 der cone embracing acrosome filament; f, acrosome filament within egg 

 proper; g-h, from two specimens of A. forbesii, successive stages which 

 preceded stages closely resembling that shown in f. 



The overall picture of sperm entry in Thyone, then, agrees 

 with the main observations in H. atra. From the very earliest stage 

 of cone formation the acrosome filament is already in contact 

 with the egg surface and becomes encased by the elevating cone. 

 As part of the sperm head, the acrosome filament enters into the 

 protoplasm of the egg proper. The entire filament enters in H. 



