BEGINNINGS OF CO-OPERATION 7 I 



towels and instruments. The procedures require as 

 rigid cleanliness as a surgical operation. Conse- 

 quently it was not surprising when I first took up 

 their study a few years ago, to have one of my frank- 

 est friends among the long-time workers on the de- 

 velopment of Arbacia, voice what was apparently a 

 common feeling among them. He asked pointedly 

 if I thought I could come into that well-worked field 

 and without long training find something they had 

 overlooked. Such frank skepticism was refreshingly 

 stimulating and added to the normal zest of bio- 

 logical prospecting. 



The shed eggs of Arhacia are about the size of 

 pin points and are just visible to the naked eye. The 

 spermatozoa are tiny things; the individual sperm 

 are invisible without a microscope although readily 

 seen when massed in large numbers. When a few 

 drops of dilute sperm suspension are added to well- 

 washed eggs, one spermatozoan unites with one q^^. 



After some fifty minutes at usual temperatures, the 

 egg divides into two cells. We call this the first 

 cleavage. Thirty or forty minutes later a second 

 cleavage takes place and thereafter cleavages occur 

 rapidly. Within a day, if all goes well, such an egg 

 will have developed into a freely swimming larva. 

 Other things being equal, (lo) the time after fer- 

 tilization to first, second and third cleavage is speeded 



