6i 4 SCIENCE PROGRESS 



assume that each specimen " inherits, in general outlines, the 

 genetic constitution of its species ; in smaller details, that of 

 its pure line ; in still more minute details, that of its indi- 

 vidual parent ; and yet may differ from this parent in some 

 hereditary points." 



The researches of the writer (Hegner, 191 8, 1919a, 19196, 

 1920) on four species of the genus Arcella are in accord with 

 those described above, but carry the investigation further, in- 

 volving the relations between nucleus, chromatin, and cytoplasm, 

 and shell characteristics. 



Two very interesting biological theories involved in these 

 studies are those of Minot (1908) and of Hertwig (1903), 

 regarding the problems of growth, cell-division, old age, senes- 

 cence, physiological degeneration, and death. These theories 



Fig. 6. 



are based on the idea that the nucleus is the dynamic centre 

 of the cell. The nuclear membrane is considered as semi- 

 permeable, allowing a constant but selected flow of materials 

 from the nucleus into the cytoplasm and vice versa. Involved 

 in the theory is also the idea expressed by Sachs in 1892, and 

 the following year by Strasburger in a slightly different form. 

 According to Sachs, the nucleus and that part of the cytoplasm 

 that falls within its " sphere of influence " make a morpho- 

 logical and physiological unit. He proposed the term " energid " 

 for such a unit. Strasburger (1893) a ^ so recognised a limit 

 to the distance through which the metabolic interchange 

 between nucleus and cytoplasm could take place, and was 

 able to show that in the cells of certain plants the ratio between 

 nuclear size and cell size was fairly constant. He accounted 

 for cell-division on theihypothesis that, when the cytoplasm had 

 increased so as to extend beyond the working sphere of the 

 nucleus, the normal ratio was regained by the division of both 

 nucleus and cytoplasm into two. The nucleo-cytoplasmic 



