consider the resultant cell as a quite new and independent organism.
It bears in the cell and nuclear matter of the penetrating
spermatozoon a part of the father's body, and in the protoplasm and
caryoplasm of the ovum a part of the mother's body. This is clear from
the fact that the child inherits many features from both parents. It
inherits from the father by means of the spermatozoon, and from the
mother by means of the ovum. The actual blending of the two cells
produces a third cell, which is the germ of the child, or the new
organism conceived. One may also say of this sexual coalescence that
the STEM-CELL IS A SIMPLE HERMAPHRODITE; it unites both sexual
substances in itself.

(FIGURE 1.23. The fertilisation of the ovum by the spermatozoon (of a
mammal). One of the many thread-like, lively spermidia pierces through
a fine pore-canal into the nuclear yelk. The nucleus of the ovum is
invisible.

FIGURE 1.24. An impregnated echinoderm ovum, with small homogeneous
nucleus (e k). (From Hertwig.))

I think it necessary to emphasise the fundamental importance of this
simple, but often unappreciated, feature in order to have a correct
and clear idea of conception. With that end, I have given a special
name to the new cell from which the child develops, and which is
generally loosely called "the fertilised ovum," or "the first
segmentation sphere." I call it "the stem-cell" (cytula). The name
"stem-cell" seems to me the simplest and most suitable, because all
the other cells of the body are derived from it, and because it is, in
the strictest sense, the stem-father and stem-mother of all the
countless generations of cells of which the multicellular organism is