Thus the size of the animal-ovum depends essentially on the quantity of yolk that has to be supplied to the egg, and this depends in the main on whether the egg is still drawing nourishment from the mother during the development of the young animal. Therefore, as a general rule, eggs which are laid, and are surrounded and protected by a shell, are usually much larger than the eggs of animals which go through their development within the body of the mother. The best known illustration of this proposition is offered by mammals and birds, animals of equally high organization and comparable in bodily size. While the eggs of birds may be as much as 15 centimetres in length, and may weigh 1½ kilogrammes, those of most mammals remain microscopically minute, and scarcely exceed a length of 0.3 millimetres. The same principle is often illustrated within one and the same small group of animals, and even in the same species. Here, again, the Daphnids or water-fleas may serve as an example. Among these there are two kinds of eggs, summer and winter eggs, of which the former go through their development into a young animal within a brood-cavity on the back of the female, while the others are liberated into the water, and are surrounded by a hard shell. The summer eggs receive more or less nourishment from the mother by the extravasation of the nutritive constituents of the blood into the brood-cavity, and they thus require a smaller provision of yolk than the winter eggs, which are thrown entirely upon their own resources. Accordingly we find that in all Daphnids the summer eggs are at least a little smaller and have less yolk than the winter eggs, as in the genus _Daphnella_ (Fig. 70, _A_ and _B_), while in some species, e.g. of _Bythotrephes_, this difference increases so much that the summer eggs are almost without yolk, and therefore very minute (Fig. 71, _B_). The reason of this lies in the fact that in this case the brood-sac is filled with a nutritive fluid rich in albuminoid substances, so that the embryo during its development is continually supplied with concentrated nourishment. This is not the case with the winter eggs, because these are liberated into the water, and we therefore find that they are of enormous size and quite filled with yolk (Fig. 71, _A_). [Illustration: FIG. 70. _Daphnella._ _A_, summer egg. _B_, winter egg. _Oe_, 'oil-globules' of the summer egg.] [Illustration: FIG. 71. _Bythotrephes longimanus._ _A_, the brood-sac (_Br_) of the female containing two winter-ova (_Wei_), on which five large sperm-cells (_sp_) are lying. _R_, dorsal surface of the animal. _Dr_, glandular layer which secretes the shell-substance. _BK_, copulatory canal. _B_, the brood-sac (_Br_) containing two summer-ova (_Sei_). Both figures under the same magnification (100).]