[Illustration: FIG. 31.--Amoeboid movements of young egg-cells, _a_, Amoeboid ovum of _Hydra_ (from Balfour, after Kleitnenberg); _b_, early ovum of _Toxopneustes variegatus_, with pseudopodia-like processes (from Balfour, after Selenka); _c_, ovum of _Toxopneustes lividus_, more nearly ripe (from Balfour, Hertwig). A1 to A4, the primitive egg-cell of a Chalk-Sponge (_Leuculmis echinus_), in four successive conditions of motion. B1 to B8, ditto of a Hermit-Crab (_Chondracanthus cornutus_), in eight successive stages (after E. von Beneden). C1 to C5, ditto of a Cat, in five successive stages (after Pflüger). D, ditto of Trout; E, of a Hen; F, of Man. The first series is taken from the _Encycl. Brit._; the second from Häckel's _Evolution of Man_.] [Illustration: FIG. 32.--Human ovum, mature and greatly magnified. (After Häckel.)] In thus saying that the ova of all animals are, so far as microscopes can reveal, _substantially_ similar, I am of course speaking of the egg-cell proper, and not of what is popularly known as the egg. The egg of a bird, for example, is the egg-cell, _plus_ an enormous aggregation of nutritive material, an egg-shell, and sundry other structures suited to the subsequent development of the egg-cell when separated from the parent's body. But all these accessories are, from our present point of view, accidental or adventitious. What we have now to understand by the ovum, the egg, or the egg-cell, is the microscopical germ which I have just described. So far then as this germ is concerned, we find that all multicellular organisms begin their existence in the same kind of structure, and that this structure is anatomically indistinguishable from that of the permanent form presented by the lowest, or unicellular organisms. But although anatomically indistinguishable, physiologically they present the sundry peculiarities already mentioned. Now I have endeavoured to show that none of these peculiarities are such as to exclude--or even so much as to invalidate--the supposition of developmental continuity between the lowest egg-cells and the highest protozoal cells. It remains to show in this place, and on the other hand, that there is no breach of continuity between the lowest and the highest egg-cells; but, on the contrary, that the remarkable uniformity of the complex processes whereby their peculiar characters are exhibited to the histologist, is such as of itself to sustain the doctrine of continuity in a singularly forcible manner. On this account, therefore, and also because the facts will again have to be considered in another connexion when we come to deal with Weismann's theory of heredity, I will here briefly describe the processes in question.