CHAP, iv.] from 1838 to 1851. 329 



division step by step ; his observations sufficed to make 

 Schleiden's theory very improbable without offering enough 

 foundation for a new one, and Schleiden did not fail to reply 

 to Unger's objections in the second edition of his 'Grundziige' 

 in 1845. 



Earlier in the same year, von Mohl published in the ' Bota- 

 nische Zeitung' the treatise on the primordial utricle which 

 has been already mentioned ; by the term primordial utricle he 

 meant partly the very thin layer of protoplasm, which in large 

 cells full of sap lines the inside of the cell-wall, and partly an 

 outer layer of the protoplasm in young cells, which are still 

 rich in that substance. It is true that the distinguishing the 

 primordial utricle was not a very important matter; but von 

 Mohl applied it with his usual thoroughness to obtaining a 

 better insight into cell-formation by calling attention (p. 289) to 

 the circumstance, that the cells of the cambium-layer between 

 the rind and the wood fit into one another and leave no inter- 

 cellular spaces ; from this he concluded that there are only two 

 possible modifications of cell-multiplication, either division of 

 cells by formation of a dividing wall or formation of cells 

 within cells ; in each of these young cells is a primordial 

 utricle, the origin of which must at least be contemporary with 

 that of the cell (cell-membrane). ' Could it then be distinctly 

 shown, that two primordial utricles exist side by side in cells, 

 which are in the act of multiplying, before a partition-wall is 

 formed between them, it would be evident that in the cambium 

 layer and at the points of the stem and root the formation of the 

 primordial utricle precedes that of the cell.' Von Mohl believed 

 that he had seen this process, but was not perfectly satisfied as 

 to the correctness of his observation ; but he continues : ' Since 

 every young cell contains a primordial utricle, this must either 

 be absorbed before a multiplication of the cell commences in 

 order to make way for two new ones formed in its stead, or the 

 old primordial utricle must separate into two.' He considered 

 the first supposition to be the probable one, rejecting Unger's 



