refraining from all speculative consideration of cases that lay
clearly before him, and his views were still undecided in 1845,
when Unger and Nägeli had already made good observations
on the formation of tissue-cells in growing organs ('Vermischte
Schriften,’ 1845, p. 336).
Schleiden's theory of cell-formation arose out of a curious mixing together of obscure observations and preconceived opinions, and reminds us indeed strongly of the old notions of Sprengel and Treviranus; it is true that he distinctly rejected their views, but he too made new cells arise from very minute granules, and his theory like theirs did not rest on any thorough course of observation.
Robert Brown, (see his Miscellaneous Writings, edited by T. T. Bennett, I.) had discovered the nucleus in the cells of the epidermis of Orchidaceous plants in 1831, and had shown that it was very generally present in the tissue-cells of Phanerogams, but had obtained no results from his discovery. The cell-nucleus lay undisturbed, till Schleiden suddenly made it the soul of his theory and the starting-point of all cell-formation. He considered that the nucleus was formed from the mucilaginous content of the cell, which he assumed on insufficient grounds to be of the nature of gum; this he called the cytoblastem, and the nucleus itself the cytoblast. As he states that his cytoblastem becomes yellow and granular in solutions of iodine, we may recognise in it our own protoplasm.
We make acquaintance with Schleiden's theory of cell-formation in its original form, if we turn to his treatise, 'Beitrage zur Phytogenesis,' (in the Archivfiir Anatomic, Physiologic, etc. von Johannes Miiller, 1838). The work begins with some remarks on the general and fundamental laws of human reason, etc., discusses the literature of cell-formation in a few lines without mentioning von Mohl's numerous observations, goes on to mention the general occurrence of the nucleus which here receives its new name, then occupies itself with gum, sugar, and starch, and at last comes to the main subject. There are
