98 Trans. Acad. Sci. of St. Louis. 



volve a remarkable transformation of the nucleus to 

 which Schleicher in 1878 gave the name of karyokinesis. In 

 certain cases, however, the simpler method of division ex- 

 ists, which is now recognized as direct division and con- 

 trasted with the indirect method of karyokinesis. The 

 indirect process is an intricate device for the purpose of 

 dividing and distributing to each of the resulting two cells 

 equal portions of a substance contained in the nucleus, termed 

 chromatin, which we have many reasons for indentifying as 

 the essential constituent of the cell, controlling its activity 

 and determining its specific nature. In indirect nuclear divi- 

 sion, therefore, the result is attained that precisely equivalent 

 portions of the chromatin are passed into the daughter-cells 

 which thus receive the same specific constitution as possessed 

 by the parent -cell. 



Not only do the cells of the body arise in this manner by 

 division of pre-existing cells, but it has been shown by the 

 painstaking labors of a host of investigators that all the cells 

 can be traced back to the fertilized egg- cell which by a suc- 

 cessive series of divisions ultimately gives rise to the vast 

 multitude of cells composing the body of the adult. This 

 process has now been followed with great accuracy in a large 

 number of cases, both animals and plants, and the genetic 

 continuity of all cells of the body has been thereby thoroughly 

 established. Nor does the process of cell-division start with 

 the cleavage of the egg, for the link between successive gen- 

 erations has been shown to consist in the fact that the ess- 



Do 



cell and the sperm-cell arise in the body of the parent by 

 division of pre-existing cells and are therefore directly derived 

 from an egg-cell of the preceding generation. The old Greek 

 doctrine of equivocal or spontaneous generation has long since 

 been discarded, and we now know that living things constitute 

 an uninterrupted series from generation to generation. 

 Although the body of the individual dies, the germ-cells live 

 on, embodying the sum-total of the race behind them and 

 giving to the generation arising from them the expression of 

 this inheritance. 



The spermatozoon discovered by Liidwig Hamm,a pupil of 

 Leeuwenhoek, in 1677, and first regarded as a parasitic ani- 



