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COOK 



which were supposed to lie hidden inside the nuclei of the 

 reproductive cells. 



If we trace back the organic series to their more simple repre- 

 sentatives we not only find that the body cells become more like 

 each other, but that the distinction between somatic or body cells 

 and reproductive cells quite fades out. When the unicellular stage 

 is reached, the problem of heredity seems largely eliminated, 

 for here reproduction consists merely in the repeated division of 

 cells into two equal parts, the close similarity of which appears 

 in no way mysterious. The difference between the higher 

 plants and animals and the lower lies in the fact that in the 

 former the cells do not repeat indefinitely the same size, shape 

 and structure, but are greatly diversified, though remaining 

 joined together in colonies or compound individual organisms. 

 Viewed in this manner it becomes apparent that there is no par- 

 ticular point at which this mechanical idea of heredity becomes 

 necessary, no definite stage where the similarity of parts of a 

 divided cell ceases to explain the facts of organic structure. 



Reproduction and growth frequently figure merely as two 

 names for the same process. Division of cells, which is repro- 

 duction among the lowest organisms, means growth in the 

 higher. The process of conjugation of cells commonly termed 

 sexual reproduction, need not be allowed to complicate the 

 question of heredity, since the same stages of gradual differ- 

 entiation can be traced among double- or conjugate-celled 

 organisms as among simple-celled. Organisms which have 

 conjugated recently do not divide differently from those which 

 have not, though they may not be able to continue to divide 

 indefinitely without conjugation. Among the higher compound 

 organisms, conjugation takes place only at the unicellular stage. 

 All the cell divisions necessary to the building up of the plant 

 or animal body must be carried on without any readjustments 

 of conjugate relations. To this limitation is doubtless due the 

 fact that as organisms increase in complexity and in special- 

 ization of tissues, conjugation becomes a more and more indis- 

 pensable preliminary to the reproduction of each new cell 

 colony, or compound individual. If, for example, there could 

 be one hundred divisions between each conjugation, this would 



