The ripe or mature egg is capable of being fertilised.
Before turning to this important fact, which, by the way, will bring us to our specially chosen type, the Echinus, a few words may be devoted to the phenomenon of “parthenogenesis,” that is to say, the possibility of development without fertilisation, since owing to the brilliant discoveries of the American physiologist, Jacques Loeb, this topic forms one of the centres of biological interest at present. It has long been known that the eggs of certain bees, lice, crayfishes, and other animals and also plants, are capable of development without fertilisation at all. Now Richard Hertwig and T. H. Morgan already had shown, that at least nuclear division may occur in the eggs of other forms--in the egg of the sea-urchin for instance--when these eggs are exposed to some chemical injuries. But Loeb[4] succeeded in obtaining a full development by treating the eggs of echinoderms with chloride of magnesium; thus artificial parthenogenesis had been discovered. Later researches have shown that artificial parthenogenesis may occur in all classes of the animal kingdom and may be provoked by all sorts of chemical or physical means. We do not know at present in what the proper stimulus consists that must be supposed here to take the place of fertilisation; it seems, of course, highly probable that it is always the same in the last resort.[5]
[4] *Amer. Journ. Physiol.* vols. iii. and iv. 1900.
[5] According to Delage (*Arch. Zool. exp.*, 3 sér. 10, 1902), it is indifferent for the realisation of artificial parthenogenesis, whether but one, or both, or neither of the “polar bodies” has been formed. But the egg must be in the first stages of maturation to the extent that the “nuclear membrane” must be already dissolved.
But enough about processes, which at present are of a highly scientific, but hardly of any philosophic interest.
By fertilisation proper we understand the joining of the male element, the spermatozoon or the spermia, with the female element, the egg. Like the egg, the spermatozoon is but a cell, though the two differ very much from one another in the relation between their protoplasm and nucleus: in all eggs it is the protoplasm which is comparatively very large, if held together with somatic cells, in the spermatozoon it is the nucleus. A large amount of reserve material, destined for the growth of the future being, is the chief cause of the size of the egg-protoplasm. The egg is quite or almost devoid of the faculty of movement, while on the contrary, movement is the most typical feature of the spermia. Its whole organisation is adapted to movement in the most characteristic manner: indeed, most spermatozoa resemble a swimming infusorium, of the type of Flagellata, a so-called head and a moving tail are their two chief constituents; the head is formed almost entirely of nuclear substance.