Protoplasm is rich in lipoids; probably it is mainly an emulsion of these and proteins. Any physical or chemical stimulus which can liquefy the lipoids causes cytolysis of the egg. The protein of the egg can really only swell or be dissolved if the condition of aggregation of the lipoid is altered by chemical or physical agencies. The mechanism of cytolysis consists in the liquefaction of the lipoids and thereupon the lipoid-free protein swells or is dissolved by taking up water.... Hence this supports Loeb’s view that membrane formation is induced by the liquefaction of lipoids.[91] [90] Loeb, J., _Über den chemischen Charakter des Befruchtungsvorgangs_, etc., Leipzig, 1908. [91] v. Knaffl, E., _Arch. f. d. ges. Physiol._, 1908, cxxiii., 279. The writer suggested that the destruction of an emulsion in the cortical layer might possibly be the essential feature of the alteration leading to membrane formation and development. It had been long observed that unfertilized starfish eggs may begin to develop apparently without any outside “stimulus,” and A. P. Mathews found that slight mechanical agitation of these eggs in sea water increased the number which developed. It has been shown in numerous experiments by Delage, R. S. Lillie, and the writer, that the substances causing development in the starfish egg are identical or closely related to those which bring about this effect in the egg of the sea urchin and in both cases the development is preceded by a membrane formation. But how can membrane formation be produced by mere agitation? It seems to me that this can be understood if we suppose that it depends upon the destruction of an emulsion in the cortical layer of the egg. It is conceivable that in the egg of certain forms the stability of this emulsion is so small that mere shaking would be enough to destroy it and thus induce membrane formation and development.[92] [92] Loeb, J., _Artificial Parthenogenesis and Fertilization_, p. 255. The durability of emulsions varies, and where an emulsion is very durable shaking has no effect, while where it is at the critical point of separating into two continuous phases a slight shaking will bring about the separation, and where the emulsion is still less durable we observe the phenomenon of a “spontaneous” parthenogenesis. Eggs like those of most sea urchins belong to the former, eggs like those of some starfish and annelids belong to the second or third type.