36 LECTURE I. 



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, cyto- 

 blastema), 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 f ). This descrip- 

 tion 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 (cytoblasi), is the one which is usually concisely 

 designated by the name of the cell-theory (more accu- 

 rately, theory of free cell-formation), a theory of deve- 

 lopment which has now been almost entirely abandoned, 

 and in support of the correctness of which not one sin- 

 gle 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 nucleo- 

 lus 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, con- 

 sisting of gum and sugar, granules of protein-compounds are seen swimming about 

 (a), among which a few larger ones are strikingly conspicuous. Around these lat- 

 ter the former are seen conglomerated into the form of a small disc (6, 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 dis- 

 tinctly recognised to be a young cell (d, e)." 



