No. 2.] ORGANIZATION OF THE EGG OF UNIO. 277 



primary formative forces. These should be matters of pure 

 description of facts observed, or inferences of a descriptive 

 nature directly drawn from these facts ; theories of potency 

 should be left to the metaphysicians. No amount of observa- 

 tion or experiment will enable us to add anything to meta- 

 physical theories of potency ; if we are to devote ourselves to 

 these, we might as well shut up our laboratories. 



It does not appear to me that in the above sense there is 

 any insoluble problem in the whole field of heredity. We may 

 hope, by sufficiently long, patient, and searching investigation, 

 some day to be able to trace the organism through all phases 

 of its metamorphoses from the Q%g of one generation to that of 

 the next, to describe the exact nature of the organization that 

 is continued throughout, to estimate at its proper value the 

 influence of external conditions at each step, and so solve the 

 old riddle of evolution and epigenesis, and that of the influence 

 of the soma on the germ. That there is continuity of organi- 

 zation we know, but what the organization that is continued 

 may be, we do not know. This is the question that presses 

 most at the present time for solution. 



The solution of the problem of differentiation would be given 

 by an exact description of every step in the process. If this 

 ideal should ever be realized in any one case, there would be no 

 further need of scientific theoties of development. But it seems 

 extremely improbable that it can ever be actually realized, 

 though I believe that it will be possible by sufficiently detailed 

 observation so closely to approximate what we call stages of 

 development to ^ach other that but a single rational theory 

 will exist capable of filling up the gaps. Therefore, observa- 

 tions such as those of Boveri ('99), Conklin ('99), and those de- 

 scribed here seem to me the surest means of approaching this 

 theory. That experimental methods will be of immense serv- 

 ice no one can doubt. The study of living material will also 

 be an indispensable method. 



In conclusion, then, I would state as my opinion that the 

 organization that is continued from one generation to another 

 consists of nuclear material in a mass of cytoplasm possessing 

 a definite orientation, the extent and nature of which must be 



