than this result was the fact declared by Nägeli in 1846, that in
all these different kinds of cell-formation it was only the external
and secondary matters that varied, while the essential part of
the process was in all cases the same, and it was soon per-
ceived that cell-formation in the animal kingdom, which was
now being more thoroughly examined, agreed in the main with
that of the vegetable kingdom, as Schwann and Kolliker had
intimated in 1839 and 1845.
It is unnecessary to give any account here of the totally different theories which Theodor Hartig and Karsten proposed about the same time. They do not rest on careful observation, and we may omit them not merely because they are rejected by the unanimous judgment of better observers, but because they had no influence upon the development of the doctrine of cell-formation, and are therefore without historical interest.
It lies in the nature of the case, that investigations into the origin and multiplication of cells should turn the attention of observers more and more to their living contents, for these are actively and immediately concerned with the formation of new cells. The various granular, crystalline, and mucilaginous portions of the contents of cells had been repeatedly observed before 1840, and Schleiden and Meyen had specially studied the 'movements of cell-sap'; but it was in the course of observations on the history of development between 1840 and 1850 that attention was first called to a substance which plays a regular part in the formation of new cells, which envelopes the cell-nucleus discovered by Robert Brown, which undergoes the most important changes as the cell grows, which forms the entire substance of swarm-spores, and the disappearance of which leaves behind it a dead framework of cell-membrane. This substance, which is much more immediately concerned with sustaining the processes of life than is the cell-wall, was seen by Schleiden in 1838 and taken for gum. It was more carefully studied by Nägeli between 1842 and 1846, and perceived by him to be nitrogenous matter. Von Mohl described it in 1844 and 1846
