Chap. IV.] from 1 838 to 1851. 321 



first part having been laid before the Paris Academy in 1 S3 1-32, 

 Mirbel distinguished three modes of cell-formation ; in the 

 germination of the spores of Marchantia new cells are formed 

 from the germ-tube and new cells again from these by a 

 similar process, much in the same way therefore as that which 

 actually occurs in the germination of Yeast-fungi ; he found 

 a second kind of cell-formation in the production of the 

 gemmae of Marchantia, where he distinctly observed the 

 successive appearance of the dividing walls, but formed an 

 erroneous idea of the proceeding on the whole ; in the further 

 development of the gemmae and in other cases of growth he 

 considered that new cells are formed between those that are 

 already present in the manner supposed in his earlier theory. 



Von Mohl's dissertation on the multiplication of vegetable 

 cells by division, published in 1835 and reprinted in ' Flora ' of 

 1837, shows how strange these processes even then appeared ; in 

 this work he expresses some doubts about Mirbel's statements, 

 but he accepts them on the whole, and only makes incidental 

 mention of his own more numerous and better observations 

 on the development of spores ('Flora,' 1833), though he had 

 there seen several cases of cell-division and free cell-formation 

 with tolerable distinctness. Adolph Brongniart (' Annales des 

 sciences naturelles,' 1827) also had observed, though imperfectly, 

 the formation of pollen-grains in their mother-cells in Cobaea 

 scandens, and Mirbel, in the appendix to the work mentioned 

 above, had given a correct description and good figures ©f the 

 formation of pollen-cells ; and yet von Mohl neglected to com- 

 pare these important observations of cases of cell-division with 

 his own ; even in 1845, when he published the latter in a revised 

 form in his ' Vermischte Schriften,' he overlooked the close re- 

 lation between the formation of those pollen-grains and spores 

 and the cell-division in Cladophora. Still this treatise of von 

 Mohl's is of great importance in the history of the theory of cell- 

 formation, because it described a case of cell-division for the first 

 time step by step and brought all the salient points into relief. 



Y 



