REPRODUCTION 81 



ble. The human egg is approxiraately a sphere 

 ■with a diameter, in the case of a large exam- 

 ple, of about one fifth of a millimeter, and with 

 a specific gravity about that of water ; conse- 

 quently its weight must be about 0.004 of 

 a milligram. The volume of the chromosomes 

 in a fertilized mouse egg has been determined 

 to be somewhat less than one thousandth of 

 the volume of the whole egg^ and, assuming 

 that this proportion hold for the human egg,^ 

 and that its chromosomes have about the same 

 specific gravity as water, the weight of this 

 material would be about 0.000,004 of a milli- 

 gram. Yet this minute amount of substance 

 is beheved to determine to a nicety that infin- 

 ity of adult traits wherein a man resembles 

 his parents. If we assume the weight of the 

 average human being to be sixty-five kilo- 

 grams, then the weight of the determining 

 material to that which is determined is as 1 to 

 16,250,000,000,000. In attempting to grasp 

 this almost inconceivable relation, it must be 

 borne in mind that the material of the chromo- 

 somes in the egg is living and that, in the 

 growth of the individual, it assimilates and in- 

 creases in volume like other living material ; 

 it is not spread through the growing body in 



