In some silk-moths (Bombycidæ) and hawk-moths (Sphingidæ), especially in the silk-moth proper (_Bombyx mori_), in _Liparis dispar_, and in quite a number of other Lepidoptera, it sometimes happens that, out of a large number of unfertilized eggs, a few will develop and produce caterpillars. This is interesting enough, but it gains increased importance through the investigations of the Russian naturalist, Tichomiroff, who succeeded in considerably increasing the number of unfertilized eggs that developed by gently rubbing them with a paint-brush, or by dipping them for a little in dilute sulphuric acid. It is thus possible to make eggs, which would not ordinarily develop without being fertilized, capable of parthenogenetic development by means of mechanical or chemical stimulus. This sounds almost incredible, but it is beyond a doubt, and it is still further corroborated by the fact that Prof. Jacques Loeb has succeeded in inciting the eggs of a sea-urchin to parthenogenetic development by means of a chemical stimulus. When he added to the sea-water in which the eggs were laid a certain quantity of chloride of magnesium the ova developed, and not only went through the process of segmentation, but even reached the stage of the quaint easel-like Pluteus larva. Quite recently Hans Winkler has made the interesting observation that from sea-urchin sperms which have been killed by heat it is possible to extract in aqueous solution a substance capable of exciting unfertilized sea-urchin eggs to development, although they only go as far as to the sixteen-cell stage. From all these results we can at least infer so much, that chemical changes and influences may determine whether the ripe ovum shall go on to embryonic development or not, and that these influences, may be very diverse in nature in different cases. I shall return later to these important facts. When we sum up the facts we have cited with reference to the reduction of the number of chromosomes, it appears that nature is, as it were, striving to keep the number constant for each species; that in germ-cells which are destined for amphimixis they are reduced to half the normal number, but that this halving of the number is suppressed where fertilization is always absent, or that the reduction to half is compensated for again in various ways, whether by subsequent fusion of the two daughter-nuclei, which have arisen from the process of reduction, or by an independent duplicating of the chromosomes in the segmentation nucleus.