SECT. 12] CYCLOSES, PHOSPHORUS, SULPHUR 1251 



that I gm. dry weight would contain 8,100,000 of these eggs, or 

 4,000,000 of those of the sand-dollar. Egg-size is variable : 



but may be averaged at 75/z, and spermatozoon size is fairly constant, 

 say 2/x, leaving the tail out of account. The cubic contents would 

 therefore be, of the egg 1 13,000 cubic /x and of the sperm 4-18 cubic fx. 

 One egg contains, therefore, 0-99 x lO"^ mgm. of total and 

 0-25 X io~® of nucleoprotein phosphorus. Calculation from the re- 

 lati\ e volumes of egg and spermatozoon, assuming the same water- 

 content, gives 218,000,000,000 sperms contained in i gm. dry weight 

 of sperm. The best values for phosphorus-content of echinoderm 

 spermatozoa are still those of Matthews, who obtained 28-6 mgm. 

 phosphorus per gm. dry weight in the case oi Strongylocentrotus. The 

 total phosphorus in one spermatozoon would thus be 0-00013 x io~^ 

 and the nucleoprotein phosphorus could hardly be more than 85 per 

 cent, of this. So the total phosphorus brought in by the spermatozoon 

 which fertilises the egg is about one ten-thousandth part of the total 

 phosphorus which is there already, a computation which agrees 

 roughly with the fact that one spermatozoon is about one thirty- 

 thousandth part of an egg in size. 



Little is known from a dynamic point of view about the sterols of 

 the echinoderm egg, although, as has been stated in Section i , there 

 is a certain amount of information about the sterols as components 

 of the unfertilised ovum. But Ephrussi & Rapkine in 1928 obtained 

 the following figures for the total unsaponifiable fraction of developing 

 Strongplocentrotus eggs : 



Hours from % dry % of the total 



fertilisation weight ether extract 



o 3'3 15-7 



12 (blastulae) 3-0 i6-i 



40 (plutei) 2-7 15-6 



From this it would seem that the total unsaponifiable fraction 

 always bears a constant relation to the total fatty acids, and as they 



