Since each daughter nucleus of a dividing blastomere has the same number of chromosomes as the original nucleus of the egg, it is clear that in a normally fertilized egg each nucleus has twice the mass of chromosomes that is contained in the nucleus of a merogonic egg, _i. e._, an enucleated fragment of protoplasm into which a spermatozoön has entered and which is able to develop. Such a fragment has only the sperm nucleus. This phenomenon of merogony was discovered by Boveri and was elaborated by Delage.[172] Boveri, in comparing the final size of the cells in normal and merogonic eggs after the cell divisions had come to a standstill, found that this size is always in proportion to the original mass of the chromatin contained in the egg; the cells of the merogonic embryo, _e. g._, the mesenchyme cells, are only half the size of the same cells in the normally fertilized embryo. Driesch furnished a further proof of Boveri’s law, that the final ratio of the mass of the chromatin substance in a nucleus to the mass of protoplasm is a constant in a given species. Driesch compared the size of the mesenchyme cells in a sea-urchin embryo produced by artificial parthenogenesis with those of a normally fertilized egg and found them half of the size of the latter. When the fertilized eggs and the parthenogenetic eggs are equal in size from the start,--which is practically the case if eggs of the same female are used,--the process of the formation of mesenchyme cells comes to a standstill when their number in the normally fertilized eggs is half as large as the final number in the parthenogenetic egg.[173] Boveri’s results as well as those of Driesch were obtained by counting the cells formed by eggs of equal size and not by simply measuring the size of the cells. It is most remarkable that certain apparent exceptions to Boveri’s law which Driesch has actually found had been predicted by Boveri. [172] Delage, Y., _Arch. Zoöl. expér._, 1899, vii., 383. [173] Driesch, H., _Arch. f. Entwcklngsmech._, 1905, xix., 648. These facts show that the growth of an organ comes to a standstill when a certain size is reached or a certain number of cells are formed. We cannot yet state why this should be, but we are able to add that the formation of a lens of normal size in the regeneration of the eye is in harmony with the phenomena in the embryo. There seems therefore no reason for stating that the regeneration of the lens cannot be explained on a purely physicochemical basis. The only justification for such a statement on the part of Wolf is that he was not in possession of the more complete set of facts now available through the work of Fischel and Uhlenhuth.