when first separated, are still naked, but they soon become surrounded by a cell-membrane " (p. 14) ; and that in higher plants, as in the pol- len of many Dicotyledons, " the contracting daughter-cells secrete cel- lulose even duriug their separation " (p. 14). Here, then, in whatever way we interpret it, the fact is that there quickly arises an outer layer different from the contained matter. But the most significant evidence is furnished by " the masses of protoplasm that escape into water from the injured sacs of Vaucheria, which often instantly become rounded into globular bodies," and of which the " hyaline protoplasm envelopes the whole as a skin" (p. 41) which " is denser than the inner and more watery substance " (p. 42). As in this case the protoplasm is but a fragment, and as it is removed from the influence of the parent-cell, this differentiating process can scarcely be regarded as anything more than the effect of physico-chemical actions : a conclusion which is sup- ported by the statement of Sachs that " not only every vacuole in a solid protoplasmic body, but also every thread of protoplasm which penetrates the sap-cavity, and finally the inner side of the protoplasm- sac which encloses the sap-cavity, is also bounded by a skin " (p. 42). If then " every portion of a protoplasmic body immediately surrounds itself, when it becomes isolated, with such a skin," which is shown in all cases to arise at the surface of contact with sap or water, this pri- mary differentiation of outer from inner must be ascribed to the direct action of the medium. Whether the coating thus initiated is secreted by the protoplasm, or whether, as seems more likely, it results from transformation of it, matters not to the argument. Either way the ac- tion of the medium causes its formation ; and either way the many varied and complex differentiations which developed cell- walls display, must be considered as originating from those variations of this phys- ically-generated covering which natural selection has taken advan- tage of.
The contained protoplasm of a vegetal cell, which has some self- mobility and when liberated sometimes performs amoeba-like motions for a time, may be regarded as an imprisoned amoeba ; and when we pass from it to a free amoeba, which is one of the simplest types of first animals, or Protozoa, we naturally meet with kindred phenomena. The general trait which here concerns us, is that while its plastic or semi-fluid sarcode goes on protruding, in irregular ways, now this and now that part of its periphery, and again withdrawing into its interior first one and then another of these temporary processes, perhaps with some small portion of food attached, there is but an indistinct differ- entiation of outer from inner (a fact shown by the frequent coalescence of pseuopodia in Rhizopods) ; but that when it eventually becomes quiescent, the surface becomes differentiated from the contents : the passing into an encysted state, doubtless in large measure due to in- herited proclivity, being furthered, and having probably been once initiated, by the action of the medium. The connexion between con-