INTRODUCTION.
ceeded in rendering the origin of young cells from nuclei within the parent-cells in the branchial cartilages very probable, the matter was decided. Cells presented themselves in the anima body having a nucleus, which in its position with regard to the cell, its form and modifications, accorded with the cytoblast of vegetable cells, a thickening of the cell-wall took place, and the formation of young cells within the parent-cell from a similar cytoblast, and the growth of these without vascular connexion was proved. This accordance was still farther shown by many details; and thus, so far as concerned these individual tissues, the desired evidence, that these cells correspond to the elementary cells of vegetables was furnished. I soon conjectured that the cellular formation might be a widely extended, perhaps a universal principle for the formation of organic substances. Many cells, some having nuclei, were already known; for example, in the ovum, epithelium, blood-corpuscles, pigment, &c. &c. It was an easy step in the argument to comprise these recognized cells under one point of view; to compare the blood-corpuscles, for example, with the cells of epithelium, and to consider these, as likewise the cells of cartilages and vegetables, as corresponding with each other, and as realizations of that common principle. This was the more probable, as many points of agreement in the progress of development of these cells were already known. C. H. Schultz had already proved the preexistence of the nuclei of the blood-corpuscles, the formation of the vesicle around the same, and the gradual expansion of this vesicle. Henle had observed the gradual increase in size of the epidermal cells from the under layers of the epidermis, towards the upper ones. The growth of the germinal vesicle, observed by Purkinje, served also at first as an example of the growth of one cell within another, although it afterwards became more probable that it had not the signi- fication of a cell, but of a cell-nucleus, and thus furnished proof that everything having the cellular form does not necessarily correspond to the cells of plants. A precise term for these cells, which correspond to those of plants, should be adopted; either elementary cells, or vegetative cells (vegetations-zellen). By still further examination, I constantly found this principle of cellular formation more fully realized. The germinal membrane was soon discovered to be composed entirely of cells, and
