1892,] Heredity and the Germ- Cells. 667 
stance and dynamic material. It is certain from this and from 
the observations of Roux that the sperm-cell is now to be 
regarded as more than a mere nucleus, that it contains both 
nuclein and para-nuclein. 
Intercellular Forces.—The forces within the different portions 
of the cell lead us to consider those which must exist between 
different cells. This is an obscure question at present; but, as 
I have observed in the close of the second lecture, it is an 
extremely important one in connection with the problem of 
heredity. As Professor Wilson writes, “My own conviction 
steadily grows that the cell is not a self-regulating mechanism 
in itself, that no cell is isolated, and that Weismann’s funda- 
mental proposition is false.” 
It is a long step between an 4 priori conviction and the dem- 
onstration by experiment of a correlation of forces between 
the cells. This seems to me a most important field of experi- 
ment. ‘We haveseen in Maupas’s work that the contact of two 
Infusoria initiates a rapid series of internal changes; we have 
only to conceive of analogous changes taking place when two 
cells are not in actual contact, as in the phenomena of previous 
fertilization referred to in my second lecture. Hertwig and 
others have shown how gravitation is related to cell activity. 
Roux has destroyed half an embryo with a hot needle in the 
first stages of segmentation and followed the other half 
through the stages of subsequent development. Another clever 
experimenter has turned fertilized ova upside down during the 
early stages of development, and shown how the protoplasmic 
pole and yolk-pole forcibly change places. Driesch has traced 
the connection and meaning of the first plane of cleavage in 
the embryos of echinoderms, and has succeeded in raising a 
small adult from half an embryo artificially separated during 
the first cleavage stage. Wilson, in the larva of Nereis, has 
shown how a certain stage of division in one group of cells 
affects all the other groups. All these experiments are in the 
line of determining the relation which exists between internal 
cell forces and other natural forces. What we must now seek 
to determine is the relation of cell to cell throughout the body, 
in connection with the phenomena of heredity. 
48 
