_The Rôle of the Nucleus_.—Experiment has shown that the nucleus presides over the nutrition, the growth, and the conservation of the cell. If, following the example of Balbiani, Gruber, Nussbaum, and W. Roux of Leipzig, we cut into two a cell without injuring the nucleus, the fragment which is denuded of the nucleus continues to perform its functions for some time in the ordinary manner, and in some measure in virtue of its former impulse. It then declines and dies. On the contrary, the fragment provided with the nucleus repairs its wound, is reconstituted and continues to live. Thus the nucleus takes a very remarkable part in the reproduction of the cell, but it is still a matter of uncertainty whether its rôle is here subordinated to that of the cellular body, or if it is pre-eminent. However that may be, it follows from this experiment that the nucleus presents all the characteristics of a vigorous vitality, and that it is in its protoplasm that the chemists should be able to find the compounds, the special albuminoids, which, _par excellence_, form living matter. § 3. THE PHYSICAL CONSTITUTION OF LIVING MATTER. THE MICELLAR THEORY. _Physical Constitution of Living Matter_.—Microscopic examination does not take us much farther. The microscope, with the strongest magnification of which it is capable at present, shows us nothing beyond these links of aligned microsomes forming the species of protoplasmic thread or mitome, whose cellular body is a confused tangle or a very tangled ball. It is not probable that direct sight can penetrate much farther than this. No doubt the microscope, which has been so vastly improved, is capable of still further improvement. But these improvements are not indefinite. We have already reached a linear magnification of 2000, and theory tells us that a magnification of 4000 is the limit which cannot be passed. The penetrating power of the instrument is therefore near its culminating point. It has already given almost all that we have a right to expect from it. We must, however, penetrate beyond this microscopic structure at which the sense of sight has been arrested. How is this to be done? When observation is arrested, hypothesis takes its place. Here there are two kinds of hypotheses, the one purely anatomical, the other physical. Anatomically, beyond the visible microsomes there have been imagined invisible hyper-microscopic corpuscles, the plastidules of Haeckel, the idioblasts of Hertwig, the pangenes of de Vries, the plasomes of Wiesner, the gemmules of Darwin, and the biophores of Weismann. Biologists who have not got all that they hoped from microscopic structure are therefore thrown back on hyper-microscopic structure.