Thus there are no fixed relations of parts and therefore no differentiations.
And it is noteworthy that in certain of the Amoebae, less excursive thanothers of the type in the movements of their substance, we see an incipientdifferentiation: sometimes there is an investing film, "delicate andevanescent," implying that an outer part which is for a short time stationary,begins to be differentiated. Perceiving, then, that this apparent exceptionis in fact a verification, we go on to observe that permanent relations ofinner and outer are followed by permanent differentiations. Elsewhere (Essays,i, 439) I have quoted from Sachs various proofs that a portion of protoplasm,whether normally detached, as in a spore, or abnormally detached, as by arupture, forthwith becoming globular, at once acquires a surface denser thanthe interior; and Kerner similarly describes the protoplasm of a zoosporeas "fixing itself and putting on a delicate cell-wall." These cases,joined with those of various Protozoa which, ceasing their active changesof form, pass into a resting stage and become enclosed in a cyst, and joinedwith the cases of Protophyta, like Sphaerella nivalis or "Red Snow,"which, in its young stage ovoid, flagellate, locomotive, and secreting askin, presently passes into a resting stage and becomes spherical and coveredby a substantial cell-membrane, yield clear evidence that in these lowesttypes there is a lapse from a more homogeneous state into a less homogeneousstate. And throughout the higher Protozoa and Protophyta, the primary contrastis between cell-membrane and cell-contents -- between the part exposed toenvironing forces and the part sheltered from them.
The transition -- the most important transition which the organic worldpresents -- between the simple forms above exemplified and those compoundforms in which a number of such are united into a colony, is well seen incertain minute algae, Pandorina and Eudorina: each being a spherically-arrangedcolony of sixteen or thirty-two members. In this first advance from unicellulartypes to multicellular types we find conformity to the general law in sofar that the hollow sphere conspicuously displays the primary contrast betweenouter and inner; a primitive amorphous cluster has undergone a marked differentiationof parts corresponding to the difference of conditions. Still more instructiveis the evidence furnished by types slightly in advance of these -- Pleodorinaand Volvox; the first consisting of some 128 cells and the second of 10,000or more. Hollow spheres like the foregoing, they present in common the significanttrait that, revolving, as they do, on a constant axis and moving forwardapproximately in the line of that axis, their two ends are exposed to slightlydifferent conditions, and the primitive homogeneity of the members of thecolony has, in consequence, lapsed into appropriate heterogeneity. Theseciliated alga-cells, whether living singly or joined into groups, severallyhave a minute red speck which is proved to be sensitive to light, and causesmotion towards it. Now in these compound forms just named, the eye-spotsare more developed in those cells forming the anterior part of the sphericalcolony-cells which also carry on more actively the nutritive function; whilethose cells which form the posterior part of the sphere, and carry on thereproductive function, have smaller eye-spots. On passing to the animal kingdom(which at its root is so little differentiated from the vegetal kingdom thatthere are unsettled disputes respecting the inclusion of the lowest formsin the one or the other) we meet with parallel illustrations. The nucleatedcell, which is the common starting point for all organisms, animal and vegetal,presents us as before with the primary contrast between inner and outer.
And as in the multicellular plants so in the multicellular animals, a likeprimary contrast is forthwith repeated in the initial clusters of cells.
