134 Historical and Critical Considerations 



the development of invisible hereditary units, but through 

 the direct passage, from the mother-cell to the daughter- 

 cells, of all the organs which compose the organism. 



The significance of this law for our hypothesis of 

 intracellular pangenesis will be discussed in the last divis- 

 ion of this Part. Here we will familiarize ourselves more 

 thoroughly with the actual basis on which it is founded. 



j. Cell-Division According to Mohl's Type 



The "Grundzuge der Anatomie und Physiologie der 

 Vegetabilischen Zelle," by Hugo von Mohl, 15 was for a 

 long time the chief source from which beginning bota- 

 nists got their knowledge. It is only Hofmeister's Pflan- 

 zenzelle (1867) and Sachs's Lehrbuch (1868) which put 

 an end to its reign, but many illustrations and statements 

 from the "Grundzuge" are still vividly remembered by 

 older botanists. 



The multiplication of cells through division is de- 

 scribed in the following manner in this book of Mohl's. 16 

 It "is introduced by changes which the primordial utricle 

 of the dividing cell undergoes, in consequence of which 

 the dividing walls develop, growing gradually inward 

 from the periphery of the cell, and separating the cell- 

 cavity into two or more cavities." We have to dis- 

 tinguish those cases where the cell-division is preceded 

 by a doubling of the nucleus, from those in which this 

 is not the case (our present poly-nucleate cells). This 

 latter, less frequent, but simpler case occurs in Conferva 

 glomerata, and therefore Mohl begins his description 

 with this alga. But even where the formation of two 

 new nuclei precedes the formation of the dividing wall, 



15 Published in Wagner's Handworterbuch der Physiologie, 1851. 

 16 Loc. cit. p. 211. 



