THE BIOLOGY OF THE CELL SURFACE 



matin demands the utilization of raw materials out of which 

 chromatin is constituted. Then the question arises: Is 

 chromatin synthesized directly from the elements of 

 which it is composed or from compounds? We know 

 that the synthesis, upon which directly or indirectly all 

 living stuff depends, comes from CO? and HOH which in 

 the presence of chlorophyl and sunlight make the simplest 

 sugars. From these by polymerization polysaccharides are 

 derived and, with N, proteins and fats are built up. In 

 other words, the protoplasmic synthesis of organic com- 

 pounds like proteins is never from elements directly but 

 from compounds in the cell. If such protoplasmic syn- 

 theses are from compounds, why not chromatin, itself a 

 conjugated protein.^ Chromatin grows because more of 

 it is made; and it is made from compounds available in the 

 cell. Even if we assume that it is made out of that material 

 which is already in the nucleus as non-chromatin, we must 

 conclude that this non-chromatin in being part of the 

 nucleus has come from the cytoplasm. 



It is clear, first, that chromatin grows; second, that It 

 can not grow in some magical way but most probably 

 grows as other cell-stuff grows — i.e., according to principles 

 governing protoplasmic syntheses as far as we to-day know 

 these, that is, from preexisting and simpler compounds. 

 The fact that during the cleavage stages of most eggs the 

 cytoplasm decreases as the chromatin Increases further 

 Indicates that chromatin is synthesized directly out of the 

 cytoplasm. Oil and yolk as such play no part in this 

 process since they tend to be segregated and the cells con- 

 taining them are the least actively dividing and those free 

 of them most productive in the elaboration of new chro- 

 matin substance. 



Thus we deal with the definite increase of a chemical 

 stuff, nuclein; a measured weight of this must mean a 

 definite weight of precursors that produce it. Briefly, in 



312 



