Each cell-division which promotes growth is followed by the enlargement of the two daughter-cells which result from it; these two daughter-elements attain the exact size of the mother-cell before division, and as soon as this size is reached a new division begins: so the growth of the whole is in the main the result of the growth of the elements. Cell-divisions during real organ-formation may behave differently, as will be described at a proper occasion.
The Egg: its Maturation and Fertilisation
We know that all the organs of an animal or plant consist of cells, and we know what acts a cell can perform. Now there is one very important organ in all living beings, which is devoted to reproduction. This organ, the so-called ovary in animals, is also built up of cells, and its single cells are called the eggs; the eggs originated by cell-division, and cell-division is to lead from them to the new adult.
But, with a very few exceptions, the egg in the ovary is not able to accomplish its functions, unless certain typical events have occurred, some of which are of a merely preparatory kind, whilst the others are the actual stimulus for development.
The preparatory ones are generally known under the name of “maturation.” The egg must be “mature,” in order that it may begin development, or even that it may be stimulated to it. Maturation consists of a rather complicated series of phenomena: later on we shall have occasion to mention, at least shortly, what happens in the protoplasm during its course; as to the nuclear changes during maturation it may be enough for our purposes to say, that there occur certain processes among the chromosomes, which lead to an extension of half of them in the form of two very small cells, the “directive cells” or “directive or polar bodies,” as they have been somewhat cautiously called.
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]
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.