THE BIOLOGY OF THE CELL SURFACE 



moves from the site of entry, the movements of those sper- 

 matozoa nearest the site slacken and cease; next those 

 farther away become immobile; finally those situated i8o° 

 away come to a standstill. These events take place before 

 membrane-separation. Often, however, the process is 

 much too rapid to allow one easily to follow it. Neverthe- 

 less I ha^'e under a good apochromatic lens followed it in 

 eggs in best condition taken from hundreds of specimens. 



The eggs of Arbacia^ in my experience, if in best condition 

 are never polyspermic. It is possible to fix eggs one second 

 after spermatozoa have been added to them; examined 

 under the microscope, each egg shows a spermatozoon 

 attached to it. If a thick sperm-suspension be added to 

 the eggs as early as one second after the first insemination, 

 no polyspermy occurs. I have also made the initial insemi- 

 nation with the heaviest sperm-concentration procurable, 

 i.e., "dry" sperm as it exudes from the male, and have 

 obtained only mono-spermic fertilization. Thus, the block 

 to polyspermy is most rapidly interposed. 



The eggs of Platynereis when laid have each one sperma- 

 tozoon attached. And yet in the body-cavity of the worm, 

 the eggs, especially those in the anterior segments, are 

 in the presence of supernumerary spermatozoa, as sections of 

 the worms made immediately after copulation reveal. In 

 all my slides of normally laid Platynereis eggs I have rarely 

 seen one egg on which two spermatozoa are attached. I 

 have never seen more than one spermatozoon within an egg. 



Surelv under the conditions of the normal insemination 

 of eggs of Platynereis one can not postulate a mechanism so 

 precise that it would distribute the spermatozoa in such 

 wise that each egg would receive only one spermatozoon. 

 Rather, within the narrow closed space of the body cavity 

 packed with eggs — even the head segment contains them — 

 prevails a situation most favorable for the aggregation of 

 supernumerary spermatozoa around each eg^. And yet 



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