the plan of its being, there is simply growth. When once the limit is attained that the species cannot pass, the organism (I mean a very rudimentary organism) reproduces itself commonly by a simple division in two halves. It perishes in doubling itself and in producing two beings, similar to itself, and having nothing to do but grow. It is by means of this bi-partition that hydras, vorticellæ, algæ, and the lowest mushrooms are generally propagated.
In the organisms that are slightly more complicated the function of reproduction tends to be specialised. The individual is no longer totally divided; it produces a bud which grows by degrees, and detaches itself from the parent organism to run in its turn through the very limited adventures of its meagre existence.
By a more advanced step in specialisation the function of reproduction becomes localised in a particular cell, an ovule, and the latter, by a series of repeated bi-partitions, develops a new individual; but it is generally necessary that the cellule destined to multiply itself by segmentation should at first dissolve by union with another cell. Through the action of various organic processes the two generating cells arrive in contact. The element which is to undergo segmentation—the female element—then absorbs the element that is simply impulsive; the element called male becomes impregnated with it, and from that moment it is fertilised, that is to say, capable of pursuing the course of its formative work.
This phenomenon, so simple in itself, of the conjugation of two cellules, is the foundation of reproduction in the two organic kingdoms as soon as the two sexes are separated. Whether the sexes are represented by distinct or united individuals, whether the accessory organic apparatus is more or less complicated, are matters of no consequence; the essential fact reappears always and everywhere of the conjugation of two cellules, with absorption, in the case of superior animals, of the male cellule by the female cellule.
The process may be observed in its most elementary form in the algæ and the diatomaceæ, said to be conjugated. To form a reproductive cellule, or spore, two neighbouring cellules each throw out, one towards the other, a prolongation.
