Produced by the repeated fissions of the primitive germ-cell, each such clusterpresently forms itself into a hollow sphere: the "cleavage cavity"being manifestly homologous with the cavity of the Vilvox-sphere.* In simple types of Metazoa, as the hydroid polyps, the blastula,being thus established in conformity with the primary contrast of conditions,there presently begins a secondary differentiation which, like that we haveseen in the Volvox but in a more pronounced manner, answers to the secondarycontrast of conditions; for this spherical assemblage of cells becomes ovoid,and by the aid of its cilia moves through the water broad end foremost: thelapse from homogeneity of form being in some cases made more pronounced bythe assumption of a sausage-shape. Simultaneously the component cells ofthe two ends become unlike in character. A far more marked differentiation,or lapse into greater heterogeneity, is seen when this single-layered spheroidof ciliated cells is changed into a double-layered spheroid by introversionof one side: a sack with the mouth sewn up and the bottom thrust in as faras it will go, serving to illustrate the relations of parts. Hence resultsthe gastrula with its ectoderm and endoderm; severally playing contrastedparts in subsequent development. So that at successive stages there is repeatedthis rise of a contrast of structures answering to a contrast of conditions-- that which occurs in the simple cell, that which occurs in the hollowsphere of such cells, and that which occurs in the double-walled sphere.
Illustrations presenting the law under another aspect -- one from eachorganic kingdom -- are instructive. The ciliated germ or planula of a Zoophytewhich, during its locomotive stage, is distinguishable only into outer andinner tissues, no sooner becomes fixed than its upper end begins to assumea different structure from its lower. The disc-shaped gemmae of the Marchantia,originally alike on both surfaces, and falling at random with either sideuppermost, immediately begin to develop rootlets on their under sides andstomata on their upper sides: a fact proving beyond question, that this primarydifferentiation is determined by this fundamental contrast of conditions.
Of course in the germs of higher organisms, the metamorphoses immediatelydue to the instability of the homogeneous, are soon masked by those due tothe assumption of the hereditary type. Even in the early stages above describedthere are to be traced modifications thus originating. Even before the primarycell-multiplication begins, there is said to be an observable distinctionbetween the two poles of the egg-cell, foreshadowing the different germ-layers.
Of course as development progresses assumption of the transmitted type ofstructure quickly obscures these primary lapses from homogeneity; thoughfor some time the fundamental relations of inner and outer are recognizablein the differentiations. But what has been said suffices to establish thealleged general truth. It is enough that incipient organisms, setting outfrom relatively homogeneous arrangements, forthwith begin to fall into relativelyheterogeneous ones. It is enough that the most conspicuous differentiationswhich they display, correspond to the most marked differences of conditionsto which their parts are subject. It is enough that the habitual contrastbetween outside and inside, which we know is produced in inorganic massesby unlikeness of exposure to incident forces, is paralleled by the firstcontrast which makes its appearance in all organic masses.
It remains to point out that in the assemblage of organisms constitutinga species, the principle enunciated is no less traceable. We have abundantmaterials for the induction that each species will not remain uniform --is ever becoming to some extent multiform; and there is ground for the deductionthat this lapse from homogeneity to heterogeneity is caused by the subjectionof its members to unlike circumstances. Tending ever to spread from its originalhabitat into adjacent habitats, each species must have its peripheral partssubject to sets of forces unlike those to which its central parts are subject,and so must tend to have its peripheral members made different from its centralmembers. §153. Among mental phenomena full establishment of the alleged lawwould involve an analysis too extensive for the occasion. To show satisfactorilyhow states of consciousness, relatively homogeneous, become heterogeneousthrough differences in the changes wrought by different external forces,would require us to trace out the organization of early experiences. Withouthere attempting this it must suffice to set down the conclusions to be drawn.
