38 THE CELL DOCTRINE. 



mentation of the egg had been observed even earlier 

 (1824) by Prevost and Dumas, all before the inves- 

 tigations of Schieiden had been made, the latter 

 author considered the apparent growing across of the 

 partition vk'alls an illusion, and that the young cells 

 escape observation in consequence of their transpar- 

 ency, until, at a late stage, their line of contact is re- 

 garded as the partition wall of the parent cell ; while 

 even Schwann states somewhat hesitatingly what is 

 now so generally admitted .* This is the cell theory of 

 Schieiden, which he assumes to be the universal law 

 for the formation of vegetable cellular tissue in the 

 phanerogamia. At that time the cryptogamia had 

 not been examined, and Schieiden had not- then ex- 

 pressed his views in reference to the cambium. 



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, 

 the stratum becomes defined externally, and a cell 

 nucleus, having a more or less sharp contour, is formed. 

 The nucleus grows by a continuous deposition of new 



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



