The contained protoplasm of a vegetal cell, which has self-mobility and when liberated sometimes performs amœba-like motions for a time, may be regarded as an imprisoned amœba; and when we pass from it to a free amœba, 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 _Monadinæ_. 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”;[5] and the genus _Scytomonas_ he says “differs from _Monas_ only in its persistent shape and accompanying greater rigidity of the peripheral or ectoplasmic layer.”[6] 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.”[7] 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-amœboid and repent state.”[8] 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_,[9] 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