CHAP, iv.] Origin of Tissues. 313 



than this result was the fact declared by Nageli 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 por- 

 tions of the contents of cells had been repeatedly observed before 

 1840, and Schleiden and Meyen had specially studied the 'move- 

 ments 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 dis- 

 covered 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 pro- 

 cesses of life than is the cell-wall, was seen by Schleiden in 

 1838 and taken for gum. It was more carefully studied by 

 Nageli between 1842 and 1846, and perceived by him to be 

 nitrogenous matter. Von Mohl described it in 1844 and 1846 



