stancy 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"; and the genus Scytomonas he says "differs from Monas only in its persistent shape and accompanying greater rigidity of the peripheral or ectoplasmic layer." Describing generally such low and minute forms, some of which 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." 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." 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, 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.
|FOOD ACCESSORIES AND DIGESTION.|
MAN, like any other animal, is so much the creature of his food—his physical perfection, his intellectual activity, and his moral tone are so dependent on the food he receives and the uses he is able to make of it in the processes of digestion and assimilation—that any accurate knowledge, founded on precise and reliable methods of investigation, of the influence on digestion and nutrition of dietetic habits must of necessity be of the most general interest.
- A Manual of the Infusoria, by W. Saville Kent. Vol. i, p. 232.
- lb. Vol. i, p. 241.
- Kent. Vol. i, p. 56.
- lb., 1. c. Vol. i, p. 57.
- The Elements of Comparative Anatomy, by T. H. Huxley, pp. 7-9.