f their progressive differentiation from the
mother-nucleus. The division of the cell-body is completed midway
between the two daughter-nuclei. In animal cells, which possess no
chemically differentiated membrane, separation is effected by simple
constriction, while in the case of plant cells provided with a definite
wall, the process begins with the formation of a cytoplasmic separating
layer.

The phenomena observed in the course of the division of the nucleus show
beyond doubt that an exact halving of its substance is of the greatest
importance. (First shown by W. Roux in 1883.) Compared with the method
of division of the nucleus, that of the cytoplasm appears to be very
simple. This led to the conception that the cell-nucleus must be the
chief if not the sole carrier of hereditary characters in the organism.
It is for this reason that the detailed investigation of fertilisation
phenomena immediately followed researches into the nucleus. The
fundamental discovery of the union of two nuclei in the sexual act was
then made (By O. Hertwig in 1875.) and this afforded a new support for
the correct conception of the nuclear functions. The minute study of the
behaviour of the other constituents of sexual cells during fertilisation
led to the result, that the nucleus alone is concerned with handing on
hereditary characters (This was done by O. Hertwig and the author of
this essay simultaneously in 1884.) from one generation to another.
Especially important, from the point of view of this conclusion, is
the study of fertilisation in Angiosperms (Flowering plants); in these
plants the male sexual cells lose their cell-body in the pollen-tube and
the nucleus only--the sperm-nucleus--reaches the egg. The cytoplasm of
the male sexual cell is therefore not necessary to ensure a transference
of hereditary characters from parents to offspring. I lay stress on the
case of the Angiosperms because researches recently repeated with
the help of the latest methods failed to obtain different results.
As regards the descendants of angiospermous plants, the same laws of
heredity hold good as for other sexually differentiated organisms; we
may, therefore, extend to the latter what the Angiosperms so clearly
teach us.

The next advance in the hitherto rapid progress in our knowledge of
nuclear division was delayed, because it was not at once recognised that
there are two absolutely different methods of nuclear division. All
such nuclear divi