1892,] Heredity and the Germ- Cells. 665 
in which the chromatin substance was of course undifferen- 
_ tiated to the eye, he treats of the nucleus as a whole without 
distinction as to chromatin and achromatin. He concludes 
that the physiological importance of the nucleus is exhibited 
in its constant interchange of materials with the remainder of 
the cell body, only through this interchange does it influence 
the cell and control its life processes. The interchange is in 
triple currents, a, from outside of cell to cytoplasm; b, from 
cytoplasm to nucleus; c, from nucleus to cytoplasm. These 
movements of interchange are the expression of life phenom- 
ena. He compares the rôle of the nucleus to that of a cell 
organoid, like chlorophyll, as not constantly present but as 
invariably necessary to activity. Thus he believes even the 
most lowly organized cells have nuclear centres, and that even 
bacteria are differentiated into nuclear and extra-nuclear areas. 
Coupled with this idea of nuclear control is the somewhat par- 
adoxical statement that the nucleus is not a dynamic centre, 
either automatic or regulating, and the conclusion that the 
nucleus alone cannot be the seat of fertilization and heredity, 
but both the nucleus and extra-nuclear protoplasm must con- 
stitute the material basis of heredity. This conclusion is in 
the direction of the general reaction of opinion which is now 
taking place against the centralization of cell-government in 
the nucleus. 
Vague as they must necessarily be, our ideas of cell forces 
are somewhat further defined by the brilliant experiments of 
the Hertwig brothers upon germ-cell physiology and pathol- 
ogy, which are full of suggestion as to the causation of abnor- 
malities in inheritance. These were begun in 1884 and were 
first directed to the influence of gravitation upon the planes 
of embryonic cell division, following up the experiments of 
Pfliiger and Rauber. In 1885 the conditions of bastard fertili- 
zation were studied ; in 1887 the causes of polyspermy or mul- 
tiple fertilization ; and in 1890 the effects of extreme heat and 
cold upon germ-cell functions." In general the conclusions 
reached were that in the normal state there exist regulating 
` NExperimentelle Untersuchungen über die Bedingungen der Bastardbefruchtung, 
Jena, 1885. See series of papers in Jenaische Zeitschrift. 
