152 



MORPHOLOGY OF FERTILIZATION 



riers through which their respective spermatozoa do not swim. 

 In these species the acrosome filament is quite long. In the ex- 

 treme case of Thyone the depth of the barrier approximates 55 

 microns and the length of the filament is sometimes even greater 

 than this. It has been suggested for some species (Colwin and 

 Colwin, 1954b; Dan, 1954) that the egg reacts only after the 

 filament of the spermatozoon makes physical contact with the 

 true egg surface or with the protoplasm immediately beneath 







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°oo °oooo_ooo 



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Fig. 36. Diagrammatic representation of successive stages of sperm en- 

 try as seen in the living egg of Holothiiria atra. (From Colwin and Colwin, 

 /. Morphol, 1955a.) 



