OF LIVING MATTER 29 



to be restricted. Without visible structure they nevertheless 

 assimilate materials from their environment, and grow ; some of 

 them constantly vary in form, and are capable of executing more 

 or less rapid movements ; though possessing no nucleus they, 

 nevertheless, are able to divide and to reproduce their kind — these 

 being, as we have seen, the fundamental properties of living 

 matter. 



Thus the cytode is the elementary vital unit and the cell is 

 a product of a higher order which may be, and constantly is, 

 developed therefrom. The first stage towards the development 

 of a typical cell occurs when the plastide undergoes some con- 

 densation of its outer layer so as to form a limiting membrane ; 

 while the second and essential stage is shown by a differentiation 

 of the contents of the primitive vital unit, and the gradual ap- 

 pearance therein of the structure known as the 'nucleus,' to 

 which an enormous importance is now attached, perhaps in some 

 respects in excess of that which it deserves. 



However Weismann and his school would interpret the fact, or 

 reconcile it with their views, there is really no room for doubt that 

 in multitudes of plastides of various kinds a nucleus appears where 

 previously there was none.' It usually reveals itself, moreover, in 

 the manner originally described by Alex. Braun in his celebrated 

 " Rejuvenescence in Nature." " During the development of repro- 

 ductive units in Hydrodictyon and other algae he observed it 

 appearing as a mere minute ' clear space ' — that is a microscopical 

 speck of matter free from granules — in the midst of a protoplasmic 

 unit. This mode of development of a nucleus the writer has 

 himself often seen in plastides in which previously there was no 

 trace of such a body. Thousands of such plastides may often be 

 seen developing in the pelUcle on a hay infusion, as will be 

 described in a subsequent chapter (see p. 196 and Figs. 24, 25). 

 Later, the nucleus also acquires a limiting membrane, and undergoes 

 other important developmental changes to which we shall subse- 

 quently have occasion to refer. The 'cell' is thus seen to be only 

 a developed form, a more visibly complex condition which a 

 simpler but already living and independent unit may assume. 



But even before the discovery of the simple Monera by Haeckel 

 it had been long previously recognised by Nageli, and also by 



■ Weismann goes so far as to say that a nucleus never arises de novo ("The 

 Evolution Theory," Transln. 1904, Vol. I, p. 309). 

 ' Transln. by Henfrey (Ray Society), 1853, p. 361. 



