shape of cells with advancing growth depend materially on
whether they enlarge equally in all parts of their circumference
or not. These considerations, obvious as they are, were now
for the first time pointed out and fully appreciated.
The reader who is already familiar with our subject will recognise in the passages adduced from Nägeli without further explanation the essential principles of the modern theory of cells, especially if he compares them with the views propounded at the same time and previously by Schleiden, Unger, and von Mohl. But, as might be expected, the further investigations, which were pursued with zeal during the succeeding twenty years and produced a considerable literature, did much to enlarge and perfect Nägeli's theory in many of its details and to correct it in some minor points; the theory itself facilitated this process by supplying a scheme to which the investigation of special questions could readily be referred. Whether the nucleus is a solid body or a vesicle, whether in the division of a mother-cell into compartments the wall of partition always grows from without inwards or is formed simultaneously over its whole surface, whether it is originally composed of two laminae or of one which is afterwards differentiated, these and many other questions were decided in course of time.
Schleiden's theory was now definitively set aside, a deeper insight was obtained into the nature of the cell, and the ideas connected with the word became broader and more profound. The knowledge of the formation of cells showed that the cell-walls, which had been hitherto regarded as the important part, are only secondary products, that the true living body of the cell is represented by its contents and especially by the protoplasm. Alexander Braun, relying on numerous researches into the lower Algae, expressed himself in 1850 ('Verjüngung,' p. 244) to the effect that it is an inconvenience that the word cell is used at one time to designate the cell with its wall, at another time the cell without its wall, or again
