No. 494] ZOOLOGICAL PROGRESS 



which consists of a mass of cytoplasm chiefly from the 

 maternal side, a centrosome from the paternal side, and 

 usually an equal number of chromosomes from each side. 

 As the offspring may resemble both father and mother, 

 it follows that the substance that is the vehicle of inheri- 

 tance is very probably the material of the chromosomes, 

 the chromatin. This chromatin carries from parent to 

 child not the vestige of an organ and is inconceivably 

 small in amount. The human egg cell is approximately 

 a sphere with a diameter of about 0.2 of a millimeter, and 

 with a specific gravity about that qf water ; consequently 

 its weight is about 0.004 of a milligram. The volume of 

 the chromatin of a fertilized mouse egg, as measured for 

 me by Mr. J. A. Long, is somewhat less than one-thou- 

 sandth of the volume of the whole egg, and, assuming 

 that this proportion holds for the human egg, and that its 

 chromatin has about the same specific gravity as water, 

 the weight of this chromatin would be about 0.000,004 of 

 a milligram. Yet this mere trace of material can influence 

 the adult substance of two identical twins to such an ex- 

 tent that their bodily configuration and actions are 

 scarcely distinguishable. If we estimate their combined 

 weights to be 130 kilograms, the chromatin of the egg 

 from which they came can be said to have influenced in 

 this profound way 32,500,000,000,000 times its original 

 weight. Of course it must be borne in mind that the chro- 

 matin of the egg is living and that in the growth of the 

 individual it assimilates and thereby increases in vol- 

 ume; the chromatin is not spread through the growing 

 body in ever-increasing dilution. But, even granting this, 

 its precision in the transmission of characteristics is cer- 

 tainly most remarkable; for when it is derived from a 

 single source, as in identical twins, its effect upon the 

 growth of the two individuals is to make them most 

 strikingly alike. It is important to observe that the chro- 

 matin of at least certain male cells is composed very 

 largely of nucleic acids, and that it is therefore very 

 probable that the chemical composition and structure of 



