NUCLEAR FUSION IN THE ASCUS. 65 



is going on, etc. — Gerassimoff has been able to check the normal course 

 of cell division in filaments of Spirogyra and to produce binucleated 

 cells or to cause the fusion of the two daughter nuclei into a single pro- 

 portionately larger nucleus. In either case the result is the same. The 

 cells with the larger nuclear mass increase in size beyond the norm for 

 the species to which they belong. These enlarged cells divide, and 

 thus filaments are formed whose cells are all above the normal in size. 

 In the binucleated cells with two fully developed nuclei the latter tend 

 ordinarily to repel each other (29, p. 73) to the extent necessary to main- 

 tain their mutually symmetrical position in the central region of the cell. 

 Still, if the cells are weakened or starving, the nuclei may approach 

 each other and apparently tend to combine. Gerassimoff concludes 

 (29, p. 'j'j') that the cells of Spirogyra possess the ability to compensate 

 for any disturbance of the quantitative equilibrium between the mass of 

 the nucleus and that of the remaining components of the protoplast by 

 varying their rate of cell division. Increase of nuclear material leads 

 to a delay of cell division and a relative diminution of nuclear material 

 in the daughter cells. On the other hand, lack of nuclear material is 

 followed by increased frequency in cell division. It should be remem- 

 bered that other methods might lead to the same result, and if the main- 

 tenance of an equilibrium between nuclear and cytoplasmic masses is 

 a fundamental necessity for the cell, it may be expected that in different 

 cases different means adapted to the special conditions of the individual 

 cells or organisms will be found in operation. 



R. Hertwig (44, 45) has reached results similar to those of Geras- 

 simoff in his experiments with cultures of certain Protozoans (Actino- 

 sphaerium, Dileptas). He finds that lack of food leads to a reduction 

 of the volume of the cytoplasm ; the nuclei shrink and a certain pro- 

 portion of them actually disintegrate. The nuclei of Actinosphaerium 

 may thus be reduced from several hundreds to one or two. In overfed 

 individuals the reverse is true. It is thus shown that the regulative 

 function is a reversible one. Experimentally achieved increase of the 

 nuclear mass in Spirogyra leads to the enlargement of the cytoplasmic 

 mass. Reduction of the cytoplasmic mass by starvation in Actino- 

 sphaerium leads to a reduction of the nuclear mass. Boveri (12) has 

 also shown by experiments, in which he fertilized both nucleated and 

 non-nucleated fragments of the eggs of sea-urchins, that those with the 

 abnormally reduced amount of nuclear material produce larvae with 

 smaller but more numerous cells. By shaking the sea-urchin eggs just 

 after fertilization he was able also to achieve the same result as did 

 Gerassimoff for Spirogyra. The daughter chromosomes after their 



