SURVEY OF CELL-LIFE. _ 179
in all cases, for, according to Schleiden, it remains persistent in most cells in the Euphorbiaceae, and the blood-corpuscles may be quoted as an example to the same effect in animals. The fact that many nuclei are developed into hollow vesicles, and the difficulty of distinguishing some of these hollow nuclei from cells, forms quite sufficient ground for the supposition that a nucleus does not differ essentially from a cell; that an ordinary nucleated cell is nothing more than a cell formed around the outside of another cell, the nucleus; and that the only difference between the two consists in the inner one being more slowly and less completely developed, after the external one has been formed around it. If this description were correct, we might express ourselves with more precision, and designate the nuclei as cells of the first order, and the ordinary nucleated cells as cells of the second order. Hitherto we have decidedly maintained a distinction between cell and nucleus; and it was convenient to do so as long as we were engaged in merely describing the observations. There can be no doubt that the nuclei correspond to one another in all cells; but the designation, “cells of the first order,” includes a theoretical view of the matter which has yet to be proved, namely, the identity of the formative process of the cell and the nucleus. This identity, however, is of the greatest importance for our theory, and we must therefore compare the two processes somewhat more closely. The formation of the cell commenced with the deposition of a precipitate around the nucleus; the same occurs in the formation of the nucleus around the nucleolus. The deposit becomes defined externally into a solid stratum: the same takes place in the formation of the nucleus. The development proceeds no farther in many nuclei, and we also meet with cells which remain stationary at the same point. The further development of the cells is manifested either by the entire stratum, or only the external part of it becoming consolidated into a membrane; this is precisely what occurs with the nuclei which undergo further development. The cell-membrane increases in its superficies, and often in thickness also, and separates from the nucleus, which remains lying on the wall; the membrane of the hollow cell-nuclei grows in the same manner, and the nucleolus remains adherent to a spot upon the wall. A trans-
