CHAPTER IV. 



HISTORY OF DEVELOPMENT OF THE CELL, FORMATION OF 

 TISSUES, MOLECULAR STRUCTURE OF ORGANISED FORMS. 



1840-1860. 



IN the period between 1830 and 1840 it had come to be 

 understood, that the old theories of cell-formation of Wolff, 

 Sprengel, Mirbel, and others, resting on indistinct perceptions 

 and not on direct and exact observation, could only give an 

 approximate idea of the formation of cells. But in the course 

 of that time really different cases of formation of new cells were 

 accurately observed by Mirbel, and more especially by von Mohl, 

 who described different modes of formation of spores, and in 

 1 83 5 the first case of vegetative cell-division. Unfortunately these 

 observations, excellent in themselves, applied to cases of cell- 

 formation which do not occur in the ordinary multiplication of 

 cells in growing organs, and von Mohl guarded himself from 

 founding a general theory of cell-formation on his observations 

 on cells of reproduction and on a growing filamentous Alga. 

 Mirbel also cautiously regarded the formation of pollen-cells 

 and that which he supposed to be the process in the ger- 

 mination of spores as cases of a peculiar kind, adhering to 

 his old theory of the origin of ordinary tissue-cells. 



Schleiden's behaviour was different. Having somewhat 

 hastily observed the free cell-formation in the embryo-sac of 

 Phanerogams in 1838, he proceeded at once to frame a theory 

 upon it which was to apply to all cases of cell-formation, and 

 especially to that in growing organs. The very positive way 



