In almost all the Daphnids the winter egg develops into a perfect animal just like that to which the summer egg gives rise, although it no longer receives any nourishment after it passes into the brood-chamber. But it receives a larger supply of yolk on this account, so that the nutritive provision within the egg is sufficient to develop the perfect animal. There is only one exception to this, and it is of special theoretical interest, because it shows more plainly than any other fact that the greater or less degree of condensation in the ontogeny depends upon the combined effect of the external conditions of life. The largest of the Daphnidæ, _Leptodora hyalina_, a beautifully transparent inhabitant of our lakes, which measures about a centimetre in length (Fig. 110), also emerges from the summer egg as a perfect animal, but from the winter egg, which floats freely in the water and has only a small provision of yolk, it emerges as a nauplius, which then undergoes larval metamorphosis before it becomes a perfect animal (Fig. 111). [Illustration: FIG. 110. The largest of the Daphnids (_Leptodora hyalina_), with summer ova (_Ei_) beneath the shell (_Sch_). _I-IX_, the appendages. _II_, the oars (second antennæ) which always remain biramose in Daphnids. _sb_, setæ. _ov_, ovaries. _Schl_, œsophagus. _Ma_, stomach. _a_, anus. _H_, heart. _Au_, eye. _nG_, natural size.] Fritz Müller concluded from the repetition of the nauplius form in all orders of Crustaceans that the primitive form of the Crustacean must have been a nauplius, and that from it all the modern Crustaceans must have evolved phyletically by the addition of segments varying in number and differentiation. Now, however, it is doubted whether there ever were nauplioid types capable of reproduction. But even if the nauplii only _represent what have been the larval_ forms from very early times, they are equally important in illustrating the relations between ontogeny and phylogeny; they at any rate represent the primitive pre-cambrian larval form from which all modern Crustaceans are derived. This is borne out not only by the facts to which we have already referred, but also by those Crustacean-groups which have diverged far from the usual Crustacean habit and type. [Illustration: FIG. 111. Nauplius larva from the winter egg of _Leptodora hyalina_; after Sars.]