the external world lie dormant, for as yet they are not needed.  But they are there, though, as we have seen in the Turtle’s egg, they were not there at the beginning.  How, then, are they formed?  We may answer, that the first function of every organ is to make itself.  The building material is, as it were, provided by the process which divides the yolk into innumerable cells, and by the gradual assimilation and modification of this material the organs arise.  Before the lungs breathe, they make themselves; before the stomach digests, it makes itself; before the organs of the senses act, they make themselves; before the brain thinks, it makes itself; in a word, before the whole system works, it makes itself; its first office is self-structure.

At the period described above, however, when the new generations of cells are just set free and have taken their place in the region where the new being is to develop, nothing is to be seen of the animal whose life is beginning there, except the filmy disk lying on the surface of the yolk.  Next come the layers of white or albumen around the egg, and last the shell which is formed from the lime in the albumen.  There is always more or less of lime in albumen, and the hardening of the last layer of white into shell is owing only to the greater proportion of lime in its substance.  In the layer next to the shell there is enough of lime to consolidate it slightly, and it forms a membrane; but the white, the membrane, and the shell have all the same quality, except that the proportion of lime is more or less in the different layers.

But, as I have said, the various envelopes of eggs, the presence or absence of a shell, and the absolute size of the egg, are accessory features, belonging not to the egg as egg, but to the special kind of being from which the egg has arisen and into which it is to develop.  What is common to all eggs and essential to them all is that which corresponds to the yolk in the bird’s egg.  But their later mode of development, the degree of perfection acquired by the egg and germ before being laid, the term required for the germ to come to maturity, as well as the frequency and regularity of the broods, are all features varying with the different kinds of animals.  There are those that lay eggs once a year at a particular season and then die; so that their existence may be compared to that of annual plants, undergoing their natural growth in a season, to exist during the remainder of the year only in the form of an egg or seed.  The majority of Insects belong to this category, as do also our large Jelly-Fishes; many others have a slow growth, extending over several years, during which they reach their maturity, and for a longer or shorter time produce broods at fixed intervals; while others, again, reach their mature state very rapidly, and produce a number of successive generations in a comparatively short time, it may be in a single season.