Unger in an enquiry into the growth of internodes (‘Botanische Zeitung,’ 1844) again declared himself as an opponent of Schleiden’s theory. He maintained first of all and erroneously that the cell-nucleus is not of general occurrence in tissue where division is taking place, but he argued rightly from the position of the cells, from the difference of thickness in their walls, and from their relative size, in favour of their multiplication by the formation of dividing walls; he noticed the part played by the cell-contents in the multiplication of cells in hairs, and asserted that merismatic cell-formation (cell-division) is the general rule in the growth of organs of vegetation, while he distinctly declared that it was not possible to bring all that is actually seen at the spots where formation of cellular tissue is taking place into agreement with Schleiden’s theory. But Unger did not observe the processes that take place in cell-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 ‘Grundzüge’ in 1845. Earlier in the same year, von Mohl published in the ‘Botanische 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 intercellular 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