7. The botanist J. Sachs was the first to definitely state that in each species the ultimate size of a cell is a constant, and that two individuals of the same species but of different size differ in regard to the number, but not in regard to the size of their cells.[168] Amelung, a pupil of Sachs, determined the correctness of Sachs’s theory by actual counts. Sachs, in addition, recognized that wherever there were large masses of protoplasm, _e. g._, in siphoneæ and other cœloblasts, many nuclei were scattered throughout the protoplasm. He inferred from this that “each nucleus is only able to gather around itself and control a limited mass of protoplasm.”[169] He points out that in the case of the animal egg the reserve material--fat granules, proteins, and carbohydrates--are partly transformed into the chromatin substances of the nuclei, and that the cell division of the egg results in the cells reaching a final size in which each nucleus has gathered around itself that mass of protoplasm which it is able to control. Morgan[170] and Driesch[171] tested and confirmed the idea of Sachs for the eggs of Echinoderms. We stated in the previous chapter that Driesch produced artificially larvæ of sea urchins of one-eighth, one-fourth, and one-half their normal size by isolating a single cleavage cell in one of the first stages of segmentation of the fertilized sea-urchin egg. He counted in each of the dwarf gastrulæ resulting from these partial eggs the number of mesenchyme cells and found that the larvæ from a one-half blastomere possessed only one-half, those from a one-fourth blastomere only one-fourth, and those from a one-eighth blastomere only one-eighth of the number of cells which a normal larva developing from a whole egg possessed. Moreover, he could show that when two eggs were caused to fuse so as to produce a single larva of double size, the gastrulæ of such larvæ had twice the number of mesenchyme cells. Driesch drew the conclusion from his observations that each morphogenetic process in an egg reaches its natural end when the cells formed in the process have reached their final size. [168] v. Sachs, J., “Physiologische Notizen,” vi., _Flora_, 1893. [169] _Ibid._, ix., 425, _Flora_, 1895. [170] Morgan, T. H., _Arch. f. Entwcklngsmech._, 1895, ii., 81; 1901, xiii., 416; 1903, xvi., 117. [171] Driesch, H., _Arch. f. Entwcklngsmech._, 1898, vi., 198; 1900, x., 361.