the Development of the Organic Cell. It 



of the cell -juices appears to be a mere phenomenon of diffusion, 

 — endosmosis co-operating on the one hand, and the property of 

 assimilation possessed by the enclosing cell-wall on the other, in 

 a continuous act of intermixing the materials concerned. 



Briicke's interesting experiments prove that, as the lightning 

 shivers the strong oak, so an excessive electrical current shatters 

 the delicate cells within the hairs of Urtica, and puts an end to 

 the interesting phenomenon of the circulation of their fluid 

 contents. 



If the lower segment of the hair be deprived of its outer coat 

 and opened, one or more of the delicate-walled cells which 

 occupy this portion swell up and protrude, whilst the rest 

 sink down from the apex into the space thus left vacant, and 

 occupy it. 



The extruded cells have frequently (PI. I. fig, 3) cellular con- 

 tents, not observable in those contained in the naked apical 

 portion ; they therefore belong to the category of true tissue- 

 cells. 



The circumstance that the integument of actual tissue-cells 

 (particularly in the lower animal organisms) is frequently as de- 

 licate as the thin walls of the cells found within the hairs of the 

 nettle, and that also in other tissue-cells (especially in the sim- 

 plest plants) it is so thick, gelatinous, and transparent that it is 

 difficult to trace their boundary, has more than once lately given 

 rise to the idea that there may be naked cells (without walls), 

 which are only mucous globules. 



In my ' Histological Researches,' before referred to, I have 

 enumerated several instances in which apparently deficient cell- 

 membranes, notwithstanding their great tenuity, could be re- 

 cognized with certainty. 



In this respect the Confervse possess a special interest ; they 

 offer peculiarly suitable conditions for making investigations 

 respecting the structure and development of the cell, as they 

 may be examined under the microscope whilst growth pro- 

 ceeds — an advantage unattainable in more complex plants or in 

 animals. 



In fact, in many species of these plants, their component 

 (tissue-) cells, in the earlier phases of development, are so very 

 thin and transparent that they are rendered appreciable only by 

 a more attentive observation of the changes in form and position 

 of the other and more solid cell-contents. The observer may 

 likewise arrive at a more ready acquaintance with the origin and 

 growth of cells by the comparative investigation of their develop- 

 ment under different conditions of nutrition— experiments and 

 observations which are indispensable also to the systematist; for 

 a change of the conditions of nutrition alters not only the phe- 



