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The contained protoplasm of a vegetal cell, which has 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 differentiation of outer from inner (a fact shown by the frequent coalescence of the pseudopodia 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 inherited proclivity, being furthered, and having probably been once initiated, by the action of the medium. The connexion between constancy of relative position among the parts of the sarcode, and the rise of a contrast between superficial and central parts, is perhaps best shown in the minutest and simplest Infusoria, the Monadinae. The genus Monas is described by Kent as “plastic and unstable in form, possessing no distinct cuticular investment; … the food-substances incepted at all parts of the periphery”;[45] and the genus Scytomonas he says “differs from Monas only in its persistent shape and accompanying greater rigidity of the peripheral or ectoplasmic layer.”[46] Describing generally such low forms, some of which are said to have neither nucleus nor vacuole, he remarks that in types somewhat higher “the outer or peripheral border of the protoplasmic mass, while not assuming the character of a distinct cell-wall or so-called cuticle, presents, as compared with the inner substance of that mass, a slightly more solid type of composition.”[47] And it is added that these forms having so slightly differentiated an exterior, “while usually exhibiting a more or less characteristic normal outline, can revert at will to a pseud-amoeboid and repent state.”[48] Here, then, we have several indications of the truth that the permanent externality of a certain part of the substance, is followed by transformation of it into a coating unlike the substance it contains. Indefinite and structureless in the simplest of these forms, as instance again the Gregarina,[49] the limiting membrane becomes, in higher Infusoria, definite and often complex: showing that the selection of favourable variations has had largely to do with its formation. In such types as the Foraminifera, which, almost structureless internally though they are, secrete calcareous shells, it is clear that the nature of this outer layer is determined by inherited constitution. But recognition of this consists with the belief that the action of the medium initiated the outer layer, specialized though it now is; and that even still, contact with the medium excites secretion of it.

A remarkable analogy remains to be named. When we study the action of the medium in an inorganic mass, we are led to see that between the outer changed layer and the inner unchanged mass, comes a surface where active change is going on. Here we have to note that, alike in the plant-cell and in the animal-cell, there is a similar relation of parts. Immediately inside the envelope comes the primordial utricle in the one case, and in the other case the layer of active sarcode. In either case the living protoplasm, placed in the position of a lining to the cuticle of the cell, is shielded from the direct action of the medium, and yet is not beyond the reach of its influences.

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Limited, as thus far drawn, to a certain common trait of those minute organisms which are mostly below the reach of unaided vision, the foregoing conclusion appears trivial enough. But it ceases to appear trivial on passing into a wider field, and observing the implications, direct and indirect, as they concern plants and animals of sensible sizes.