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. These prolongations meet, and their sides absorb each other at the point of contact; then the protoplasms of the two elements mingle, and at length the two cellules melt into a single reproductive cellule (_Spirogyra longata_). Between this marriage of two lower vegetal cellules, which realises to the letter the celebrated biblical words, “they shall be one flesh,” or rather one protoplasm, and the fundamental phenomenon of fecundation in the superior animals, including man, there is no essential dissimilarity. The ovule of the female and the spermatozoon of the male become fused in the same manner, with this difference only, that the feminine cellule, the ovule, preserves its individuality and absorbs the masculine cellule, or is impregnated by it. But, simple as it is, this phenomenon of fecundation is the sole reason of the duration of bi-sexual species. Thanks to it, organic individuals that are all more or less ephemeral, “Et, quasi cursores, vitaï lampada tradunt.” (_Lucretius_, ii. 78.) For many organised beings reproduction seems in reality the supreme object of existence. Numbers of vegetables and of animals, even of animals high in the series--as insects--die as soon as they have accomplished this great duty. Sometimes the male expires before having detached himself from the female, and the latter herself survives just long enough to effect the laying of eggs. Instead of laying eggs, the female cochineal fills herself with eggs to such a degree that she dies in consequence, and the tegument of her body is transformed into a protecting envelope for the eggs.