UTRICULAR STRUCTURES. 167 



so far as to enable us to distinguish the organic molecules, 

 there will always be still entire stages, in the development 

 of membranes, in which it will be impossible to distinguish 

 them from the homogeneous contents lying close to them, 

 and refracting light in a similar manner, and in which 

 their presence or absence must be decided on upon other 

 grounds. 



The matter stands in a similar position in regard to 

 cells. The relation of contents and membrane is evident 

 in fully-formed cells, and it may be carried back by a 

 conclusion from analogy to the young cells. The young 

 cells in the embryo-sac, if they possess homogeneous and 

 not granular contents, often do not allow of our perceiv- 

 ing the membrane for a long time. Free germ-cells 

 (spores) of Algae, Eungi, and Lichens, mostly attain a 

 considerable size (at least as considerable as that of the 

 nucleus mentioned by Schleiden), without the possibility 

 of seeing the least sign of membrane, and yet they are 

 young cells, to which no one will deny a membrane.* 

 Moreover, cells occur also, about the size of young nuclei, 

 on which no membrane is visible in their lifetime. Among 

 these are some species of Protococcus, of Palmella, and of 

 other Palmelleae. 



If, then, it is impossible to distinguish the membrane 

 from the contents in young free ceils, or in small free 

 cells generally, on the other hand the membrane is usually 

 visible in parietal cells at the moment when the cell 

 originates, and in those very places where the apposition 

 of the secondary cells forms a septum. The same occurs 

 in nuclei which originate by the division of a parent- 

 nucleus. Here, too, as in the division of cells, a septum 

 is visible, formed by the two meeting membranes. f 



The comparison of those free cells is especially neces- 

 sary here, in which membrane and contents cannot be 

 clearly distinguished even in the fully- developed con- 



* See pages 95-6 of this volume. 



f Nagcli on Cells, Part I, Hay Transl. 1845, pp. 231, 246. 



