Fig. 66. Spermatozoa of various Daphnids. a, Sida.
b, Bythotrephes. c, Daphnella. d, Moina paradoxa. e,
Moina rectirostris. f, Eurycercus lamellatus. g, Alonella
pygmæa. h, Peracantha truncata. All magnified 300
times.

It is remarkable how different the sperm-cells sometimes are in quite nearly related species of Daphnids, as a glance at Fig. 66 will show; and, on the other hand, how similar they may be in two species which belong to different families, like Bythotrephes longimanus (b), and Daphnella hyalina (c). The last fact may be explained as an adaptation to similar conditions of fertilization. Both species have effective copulatory organs, and their large delicate sperm-cells must immediately adhere when they come into contact with the shell-less ovum, and penetrate into it by means of amœboid processes. Conversely, the difference between sperm-cells of allied species like Sida crystallina (a), Moina rectirostris (e) and M. paradoxa (d) is related to different adaptations to nearly the same conditions of fertilization. In Sida (Fig. 66 a) the large flat sperm-cells, with their fringed ends and their large soft surface, adhere easily to the ova, and the same end is attained in Moina rectirostris by means of stiff radiating processes, while in the nearly related species, Moina paradoxa, the male cell (d) resembles an Australian boomerang and presses in like a wedge between the ova and the wall of the brood-sac.

Fig. 67. Spermatozoa of various animals, after Ballowitz, Kölliker, and vom Rath. 1, man. 2, bat (Vesperugo). 3, pig. 4, rat. 5, bullfinch. 6, newt. 7, skate (Raja). 8, beetle. 9, mole-cricket (Gryllotalpa). 10, freshwater snail (Paludina). 11, sea-urchin. Much magnified.

In Fig. 67 a small selection of animal male cells is figured, all of which take the form of sperm-threads or spermatozoa, and yet they differ very much from one another in detail. It would undoubtedly be of great interest to follow out these minute adaptations of the sperm-cells to the conditions of fertilization, and to demonstrate that their size, and especially their form, in the different species of animals are adjusted to the special constitution of the ovum, its envelope, and its micropyles, and to the ease or difficulty with which it can be reached; but much information must be forthcoming before we can even suggest, for instance, why the sperm-cell of the salamander is so enormously long, large, and pointed at the head, while that of Man (Fig. 67, 1) is comparatively short, with broad, flat head and a recently discovered minute apex; or why those of Man and many fishes (such as Cobitis) should be so much alike, and so on. From many sides, however, we are led to conclude that even down to the minutest details nothing is in vain, and that everything depends on adaptation.

In general, even the peculiarities of form already indicate this; thus the spirally coiled structure of the head, which is especially well developed in the spermatozoa of birds (Fig. 67, 5), in those of the skate (7), and of the freshwater snail (Paludina) (10), works like a corkscrew, and makes it possible for the spermatozoon to pierce through the resistant envelope of the ovum. Similarly, the sharply pointed head of the insect spermatozoon (Fig. 67, 8 & 9) seems adapted for slipping through the minute pre-formed micropyle in the hard egg-shell.

Of the detailed and complicated structure of spermatozoa we have only recently been made aware through the increasing perfection of the microscope and of technical methods of investigation. Fig. 68 shows one after a diagrammatic figure by Wilson. We see the apical point (sp) for boring into the ovum, the nucleus (n) surrounded by a thin layer of protoplasm, which together form the head, then the middle portion (m) which contains the 'centrosome' (c), and the 'tail' or 'flagellum' which effects the movement of the whole and which itself possesses a complex structure with an 'axial filament' (ax) and an enveloping layer, the latter often drawn out into a spirally twisted, undulating membrane of the most extreme delicacy, as is most clearly seen in the newt (Fig. 67, 6).

Fig. 68. Diagram of a
spermatozoon, after Wilson.
sp, apical point.
n, nucleus.
c, centrospere.
m, middle piece.
ax, axial filament.
e, terminal filament.