The observations of Nussbaum and those who repeated his experiments showed that although two different structures are required, not the whole mass of an infusorian is needed to maintain its life. The question then arose: How small a fraction of the original cell is required to permit the full maintenance of life? The writer tried to decide this question in the egg of the sea urchin. He had found a simple method by which the eggs of the sea urchin (_Arbacia_) can easily be divided into smaller fragments immediately after fertilization. When the egg is brought from five to ten minutes after fertilization (long before the first segmentation occurs) into sea water which has been diluted by the addition of equal parts of distilled water, the egg takes up water, swells, and causes the membrane to burst. Part of the protoplasm then flows out, in one egg more, in another less. If these eggs are afterward brought back into normal sea water those fragments which contain a nucleus begin to divide and develop.[141] It was found that the degree of development which such a fragment reaches is a function of its mass; the smaller the piece, the sooner as a rule its development ceases. The smallest fragment which is capable of reaching the pluteus stage possesses the mass of about one-eighth of the whole egg. Boveri has since stated that it was about one twenty-seventh of the whole mass. Inasmuch as only the linear dimensions are directly measurable, a slight difference in measurement will cause a great discrepancy in the calculation of the mass. Driesch’s results disagree with the statement of Boveri and support the observation of the writer. [141] Loeb, J., _Arch. d. f. ges. Physiol._, 1893, lv., 525. If we raise the question why such a limit exists in regard to the divisibility of living matter, it seems probable that only those fragments of an egg are capable of development into a pluteus which contain a sufficient amount of material of each of the three layers. If this be correct, it would certainly not suffice to mix the _chemical_ constituents of the egg in order to produce a normal embryo; this would require besides the proper chemical substances a definite arrangement or structure of this material. The limits of divisibility of a cell seem therefore to depend upon its physical structure and must for this reason vary for different organisms and cells. The smallest piece of a sea-urchin egg that can reach the pluteus stage is still visible with the naked eye, and is therefore considerably larger than bacteria or many algæ, which also may be capable of further division.