6o FRANCIS O. SCHMITT 



environment provided by the orderly action of embryological organizers 

 upon the reacting system as a whole. Further progress requires a frontal 

 attack on the molecular structure of the chromosome with all the physical 

 and chemical tools at our disposal. But the investigator who ventures 

 in this difficult field must have a proper appreciation of the indetermi- 

 nacies involved. These stem largely from the fact that chromatin re- 

 moved from a nucleus for analytical study may be but a skeleton of the 

 biological entity as it exists in the living cell. This should not deter him 

 from making structural and chemical studies on chromosomal material 

 isolated in whatever form is most convenient for the purpose, but the 

 complexities imposed by the ecological factors should warn against 

 oversimplified generalizations which might be derived from such partial 

 systems. 



