210 KANSAS UNIVERSITY SCIENCE BULLETIN. 



of the same nature. Here, where vegetative reproduction may 

 be carried on for long periods of time, there are forms that 

 suffer practically no variation at all under these conditions ; but 

 allow them to propagate sexually and variation occurs as usual. 

 In a self-fertilized plant the material comes from one source in 

 both cases, but in vegetative propagation the same double set 

 of chromosomes reproduces itself constantly unchanged; but 

 in sexual reproduction there is the interaction of chromosomes 

 in synapsis and the formation of a new cytoplasm that is lack- 

 ing otherwise. The fundamental importance of the germ-cell 

 organization is thus indisputably proved. 



With the chromosomes in the role of character determinants, 

 how then may we regard the operation of the cell parts? We 

 must, in the first place, I think, consider the cyclical character 

 of cell division. Nucleus and cytosome are physically and 

 chemically unlike structures, separated by a thin membrane. 

 There can be no doubt that the ordinary phenomena of osmosis 

 find a place with consequent interchange of materials. After 

 a cell division the cytosome grows, the nucleus grows, and the 

 chromatin doubles its volume. New material has been added 

 and transformed into the likeness of the old. Experiments 

 teach us that the presence of the nucleus is required for the 

 operation of these changes. Then comes another mitosis. The 

 nucleus as a discrete body disappears and its protoplasm 

 merges with that of the c>i;osome. Thereupon there is formed 

 the familiar bipolar figure and the chromosomes are accurately 

 divided along a plane established before the breaking down of 

 the nucleus. This occurs in the spermatogonia, where the cyto- 

 plasm is reduced in amount to such a degree that so minute a 

 spindle is produced that it can scarcely be distinguished among 

 the chromosomes. The division of the chromosomes is the final 

 effort of these cells. That such a process would produce equiv- 

 alent daughter-cells seems obvious. A similar occurrence ob- 

 tains in the reproduction of somatic cells, but there is an im- 

 portant difference between the two categories. In the division 

 of the spermatogonia there is a constant increase of the chro- 

 matin at the expense of the cytoplasm, resulting in cells a very 

 large proportion of which individually is chromatin, while in 

 somatic mitoses the cytoplasm enlarges disproportionately to 

 the nucleus and its chromatin. This cytomorphosis seems to 

 exhaust the possibilities of differential interchange between 

 nucleus and cytosome, and fixes the character of the cell. 



