FIGURES 1.26 AND 1.27. Impregnation of the ovum of the sea-urchin.
(From Hertwig.) In Figure 1.26 the little sperm-nucleus (sk) moves
towards the larger nucleus of the ovum (ek). In Figure 1.27 they
nearly touch, and are surrounded by the radiating mantle of
protoplasm.)

Hence the one essential point in the process of sexual reproduction or
impregnation is the formation of a new cell, the stem-cell, by the
combination of two originally different cells, the female ovum and the
male spermatozoon. This process is of the highest importance, and
merits our closest attention; all that happens in the later
development of this first cell and in the life of the organism that
comes of it is determined from the first by the chemical and
morphological composition of the stem-cell, its nucleus and its body.
We must, therefore, make a very careful study of the rise and
structure of the stem-cell.

The first question that arises is as to the two different active
elements, the nucleus and the protoplasm, in the actual coalescence.
It is obvious that the nucleus plays the more important part in this.
Hence Hertwig puts his theory of conception in the principle:
"Conception consists in the copulation of two cell-nuclei, which come
from a male and a female cell." And as the phenomenon of heredity is
inseparably connected with the reproductive process, we may further
conclude that these two copulating nuclei "convey the characteristics
which are transmitted from parents to offspring." In this sense I had
in 1866 (in the ninth chapter of the General Morphology) ascribed to
the reproductive nucleus the function of generation and heredity, and
to the nutritive protoplasm the duties of nutrition and adaptation.