connection between the three coexistent cell-constituents was long thought to be on this wise: that the nucleolus was the first to shew itself in the development of tissues, by separating out of a formative fluid {blastema, cytoblastema), that it quickly attained a certain size, that then fine granules were precipitated out of the blastema and settled around it, and that about these there condensed a membrane. That in this way a nucleus was completed, about which new matter gradually gathered, and in due time produced a little membrane (the celebrated watch-glass form, fig. 4, d).
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This description of the first development of cells out of free blastema, according to which the nucleus was regarded as
preceding the formation of the cell, and playing the part of a real cell- former (cytoblast), is the one which is usually concisely designated by the name of the cell-theory (more accurately, theory of free cell-formation),—a theory of development which has now been almost entirely abandoned, and in support of the correctness of which not one single fact can with certainty be adduced. With respect to the nucleolus, all that we can for the present regard as certain, is, that where we have to deal with large and fully developed cells, we almost constantly see a nucleolus in them ; but that, on the contrary, in the case of many young cells it is wanting.
You will hereafter be made acquainted with a series
Fig. 4. From Schleiden, 'Grundziige der wiss. Botanik,' I, fig. 1. "Contents of the embryo-sac of Vicia faba soon after impregnation. In the clear fluid, consisting of gum and sugar, granules of protein-compounds are seen swimming about (a), among which a few larger ones are strikingly conspicuous. Around these latter the former are seen conglomerated into the form of a small disc (b, c). Around other discs a clear, sharply defined border may be distinguished, which gradually recedes farther and farther from the disc (the cytoblast), and, finally, can be distinctly recognised to be a young cell (d, e)."
