and tail. The "head" (Figure 1.20 k) is merely the oval nucleus of the
cell; the body or middle-part (m) is an accumulation of cell-matter;
and the tail (s) is a thread-like prolongation of the same.

Moreover, we now know that these spermatozoa are not at all a peculiar
form of cell; precisely similar cells are found in various other parts
of the body. If they have many short threads projecting, they are
called ciliated; if only one long, whip-shaped process (or, more
rarely, two or four), caudate (tailed) cells.

Very careful recent examination of the spermia, under a very high
microscopic power (Figure 1.22 a, b), has detected some further
details in the finer structure of the ciliated cell, and these are
common to man and the anthropoid ape. The head (k) encloses the
elliptic nucleus in a thin envelope of cytoplasm; it is a little
flattened on one side, and thus looks rather pear-shaped from the
front (b). In the central piece (m) we can distinguish a short neck
and a longer connective piece (with central body). The tail consists
of a long main section (h) and a short, very fine tail (e).

The process of fertilisation by sexual conception consists, therefore,
essentially in the coalescence and fusing together of two different
cells. The lively spermatozoon travels towards the ovum by its
serpentine movements, and bores its way into the female cell (Figure
1.23). The nuclei of both sexual cells, attracted by a certain
"affinity," approach each other and melt into one.

The fertilised cell is quite another thing from the unfertilised cell.
For if we must regard the spermia as real cells no less than the ova,
and the process of conception as a coalescence of the two, we must