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