THE CELL DOCTRINE. 33 



of formation of new cells seems to have been con- 

 ceived by him. For although the multiplication of 

 cells by fissiparous division of previously existing 

 cells, had been demonstrated by Mirbel,* and con- 

 firmed by Von Mohl,f before the investigations of 

 Schleiden had been made, the latter author considered 

 the apparent growing across of the partition cells an 

 illusion, and that the young cells escape observation 

 in consequence of their transparency, until, at a late 

 stage, their line of contact is regarded as the partition 

 wall of the parent cell; while even Schwann states 

 somewhat hesitatingly what is now so generally ad- 

 mitted. J This is the cell theory of Schleiden, which 

 has also been properly called the theory of free cell 

 formation, since it involves a spontaneous generation 

 of the cell. 



The merit of Schwann consisted in applying this 

 theory to animal tissues, his conclusions being based 

 upon the study of the formation of the chorda dor- 

 salis, and cartilage, and a comparison of their cells 

 with those of vegetable tissues. Thus, in a cyto- 

 blastema, either structureless or minutely granulous, 

 "a nucleolus is first formed ; around this a stratum 

 of substance is deposited, usually minutely granulous, 

 but not yet sharply defined on the outside. As new 

 molecules are constantly being deposited in this stra- 

 tum between those already present, and as this takes 

 place within a precise distance of the nucleolus only, 



* Mirbel, Kecherches sur la Marchantia, 1833. 

 f Von Mohl, Entwicklung und Bauder Sporen der Kryptogam. 

 Gew., Flora, 1833. 



J Schwann, op. citat. Introduction, p. 4. 



