196 THE SCIENCE OF BIOLOGY 



the cards in the hands, while knowing something of the dealing, 

 but not knowing the organization of the pack. If it is 

 found that adult qualities appear in a certain manner, their 

 probable arrangement before the dealing, that is to say the 

 development, can be inferred. 



An amazing result of the recent experimental work upon 

 the heredity of adult characters is that the knowledge thus 

 gained enables us to picture, without seeing, certain char- 

 acteristics in the organization of the germ-cells, much as the 

 chemist pictures the organization of molecules. There is, 

 however, one respect in which the biologist, who seeks to 

 understand the germ-cells has an advantage over the chem- 

 ist who postulates invisible structures. There exists within 

 the nucleus of ovum and spermatozoon, as in all other cells, 

 a visible substance, known as chromatin and appearing 

 at the time of cell division in the form of bodies, the chro- 

 mosomes (Fig. 13), which are constant, both in number and 

 appearance for any given species. The behavior of these 

 chromosomes, as seen by the microscope, is so specifically 

 related to the end results of heredity as to virtually identify 

 them with the mechanism of transmission for certain adult 

 qualities through the germ, and hence to suggest the prob- 

 able organization of the germinal substance. 10 It is thus 

 possible to attack the problem of development at its two 

 extremes; and, now that we understand the situation to 



10 Morgan, T. H., "The Mechanism of Mendelian Heredity," 1915. This 

 volume is a current summary of conclusions reached by Professor Morgan and 

 his students. The work, which is still in progress, has already yielded results 

 of such importance that it is clearly the most comprehensive attack which 

 has been yet made upon the problem of heredity. It is important in relation 

 to the problem of preformation, because certain postulates can now be made 

 regarding the organization within germ-cells. If the results are sustained and 

 extended in correlation with the work of the cytologists, a supra-molecular 

 organization of the germ may soon be accepted in a manner comparable to 

 the way in which the chemist accepts his working hypothesis of molecular 

 organization. Development will then become the problem of how a germ with 

 a given organization develops into a given adult organism how the dealing 

 is accomplished. 



