8 INTRODUCTION. 



ceeded in rendering the origin of young cells from nuclei 

 within the parent-cells in the branchial cartilages very pro- 

 bable, the matter was decided. Cells presented themselves in 

 the animal 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 con- 

 cerned these individual tissues, the desired evidence, that these 

 cells correspond to the elementary cells of vegetables was fur- 

 nished. 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, epi- 

 thelium, 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. Sclmltz 

 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 mem- 

 brane was soon discovered to be composed entirely of cells, and 



