"impregnation rise" (Figure 1.25 A). The spermatozoon then bores its
way into this with its head, the tail outside wriggling about all the
time (Figure 1.25 B, C). Presently the tail also disappears within the
ovum. At the same time the ovum secretes a thin external yelk-membrane
(Figure 1.25 C), starting from the point of impregnation; and this
prevents any more spermatozoa from entering.

Inside the impregnated ovum we now see a rapid series of most
important changes. The pear-shaped head of the sperm-cell, or the
"head of the spermatozoon," grows larger and rounder, and is converted
into the male pro-nucleus (Figure 1.26 s k). This has an attractive
influence on the fine granules or particles which are distributed in
the protoplasm of the ovum; they arrange themselves in lines in the
figure of a star. But the attraction or the "affinity" between the two
nuclei is even stronger. They move towards each other inside the yelk
with increasing speed, the male (Figure 1.27 s k) going more quickly
than the female nucleus (e k). The tiny male nucleus takes with it the
radiating mantle which spreads like a star about it. At last the two
sexual nuclei touch (usually in the centre of the globular ovum), lie
close together, are flattened at the points of contact, and coalesce
into a common mass. The small central particle of nuclein which is
formed from this combination of the nuclei is the stem-nucleus, or the
first segmentation nucleus; the new-formed cell, the product of the
impregnation, is our stem-cell, or "first segmentation sphere" (Figure
1.2).

(FIGURE 1.25. Impregnation of the ovum of a star-fish. (From Hertwig.)
Only a small part of the surface of the ovum is shown. One of the
numerous spermatozoa approaches the "impregnation rise" (A), touches
it (B), and then penetrates into the protoplasm of the ovum (C).