As the size of the two gametes is so disproportionate in most cases that the oosphere may be millions of times bigger than the sperm, and the latter at its entrance into the oosphere entirely escape unaided vision, the term "egg" is applied in lax speech, both (1) to the female cell, and (2) to the oosperm, the latter being distinguished as the "fertilised egg," a survival from the time when the union of _two_ cells, as the essence of the process, was not understood. We know that in _many_ cases, and have a right to infer that in _all_, chemiotaxy plays an important part in attracting the pairing-cells to one another. In Mammals and Sauropsida there seems also to be a rheotactic action of the cilia lining the oviducts, which work downwards, and so induce the sperms to swim upwards to meet the ovum, a condition of things that was most puzzling until the nature of rheotaxy was understood. A remarkable fact is that equal gametes rarely appear to be attracted by members of the same brood, though they are attracted by those of any other brood of the same species.[50] It may well be that each brood has its own characteristic secretion, or "smell," as it were, slightly different from that of other broods, just as every dog has his, so easily recognisable by other dogs; and that the cells only react to different "smells" to their own. Such a secretion from the surface of the female cell would lessen its surface tension, and thereby render easier the penetration of the sperm into its substance. As a rule, one at least of the pair-cells is fresh from division, and it would thus appear that the union of the nuclei is facilitated when one at least of them is a "young" one. Of the final mechanism of the union of the nuclei, we know nothing: they may unite in any of the earlier phases of mitosis, or even in the "resting state." A fibrillation of the cytoplasm during the process, radiating around a centrosome or two centrosomes indicates a strained condition.[51] {35}REGENERATION.—Finally, experiments have been made by several observers as to the effects of removing parts of Protozoa, to see how far regeneration can take place. The chief results are as follows:—