impregnation. Thus is prevented the doubling of the chromatin in the
coalescence of the two nuclei at conception. As the two polar cells
are extruded and lost, and have no further part in the fertilisation
of the ovum, we need not discuss them any further. But we must give
more attention to the female pro-nucleus which alone remains after the
extrusion of the polar cells and the dissolving of the germinal
vesicle (Figure 1.23 e k). This tiny round corpuscle of chromatin now
acts as a centre of attraction for the invading spermatozoon in the
large ripe ovum, and coalesces with its "head," the male pro-nucleus.
The product of this blending, which is the most important part of the
act of impregnation, is the stem-nucleus, or the first segmentation
nucleus (archicaryon)--that is to say, the nucleus of the new-born
embryonic stem-cell or "first segmentation cell." This stem-cell is
the starting point of the subsequent embryonic processes.

Hertwig has shown that the tiny transparent ova of the echinoderms are
the most convenient for following the details of this important
process of impregnation. We can, in this case, easily and successfully
accomplish artificial impregnation, and follow the formation of the
stem-cell step by step within the space of ten minutes. If we put ripe
ova of the star-fish or sea-urchin in a watch glass with sea-water and
add a drop of ripe sperm-fluid, we find each ovum impregnated within
five minutes. Thousands of the fine, mobile ciliated cells, which we
have described as "sperm-threads" (Figure 1.20), make their way to the
ova, owing to a sort of chemical sensitive action which may be called
"smell." But only one of these innumerable spermatozoa is
chosen--namely, the one that first reaches the ovum by the serpentine
motions of its tail, and touches the ovum with its head. At the spot
where the point of its head touches the surface of the ovum the
protoplasm of the latter is raised in the form of a small wart, the