In an able series of later works Whitman has followed out the suggestion made in his paper of 1878, already cited, pointing out how essential a part is played in development by the cytoplasm and insisting that cytoplasmic pre-organization must be regarded as a leading factor in the ontogeny. Whitman's interesting and suggestive views are expressed with great caution and with a full recognition of the difficulty and complexity of the problem. From his latest essay, indeed (94), it is not easy to gather his precise position regarding the theory of cytoplasmic localization. Through all his writings, nevertheless, runs the leading idea that the germ is definitely organized before development begins, and that cleavage only reveals an organization that exists from the beginning. " That organization precedes cell-formation and regulates it, rather than the reverse, is a conclusion that forces itself upon us from many sides." "The organism exists before cleavage sets in, and persists throughout every stage of cell-multiplication." 2 In so far as this view involves the assumption that the organization of the egg-cytoplasm at the beginning of cleavage is a primordial character of the egg, Whitman's conception must, I think, be placed on the side of the localization theory ; but his point of view can only be appreciated through a study of his own writings.
All of these views, excepting those of Roux, lean more or less distinctly towards the conclusion that the cytoplasm of the egg-cell is from the first mapped out, as it were, into regions which correspond with the parts of the future embryonic body. The cleavage of the ovum does not create these regions, but only reveals them to view by marking off their boundaries. Their topographical arrangement in the egg does not necessarily coincide with that of the adult parts, but only involves the latter as a necessary consequence — somewhat as a picture in the kaleidoscope gives rise to a succeeding picture composed of the same parts in a different arrangement. The germinal localization may, however, in a greater or less degree, foreshadow the arrangement of adult parts — for instance, in the egg of the tunicate or cephalopod, where the bilateral symmetry and anteroposterior differentiation of the adult is foreshadowed not only in the cleavage stages, but even in the unsegmented egg.
By another set of writers, such as Roux, De Vries, Hertwig, and Weismann, germinal localization is primarily sought not in the cytoplasm, but in the nucleus ; but these views can best be considered after a review of the idioplasm hypothesis, to which we now proceed.