THE BEGINNINGS OF LIFE. 217 



cell-coat, as a product of cellular activity, always stands 

 in inverse proportion to the physiological activity of 

 the cell. In youth, thin, soft, and extensible, the cell 

 coat allows abundant nutrition and advancing growth ; 

 subsequently, thickened and therewith hardened by the 

 deposit of lamellx 1 , it compresses the contents within 

 continually narrower boundaries, more and more ex- 

 cludes intercourse with the external world, and puts 

 a term to growth.' 



Taking that view of the case, therefore, which would 

 alone seem tenable in our present state of knowledge, 

 it could not be imagined that any changes occurring 

 in a simple living unit, or plastide, would be essen- 

 tially altered in character because its external layers 

 had become condensed into a so-called cell-membrane. 

 It is useless, also, to resort to the nucleus as an element 

 possessing a mysterious power of its own, and to attri- 

 bute, as was formerly the case, all the important phe- 

 nomena occurring within a Cell to the effects of its 

 influence. We are told by Nageli 2 that whole families 

 of plants are devoid of anything like a nucleus, and 



1 This more especially refers to the thickening and condensation of 

 the wall which takes place in many vegetable cells. 



2 Speaking of the occurrence of this previously supposed necessary 

 element of the cell, Braun says (loc. cit. p. 1 74) : ' Nageli's extensive 

 researches have demonstrated its occurrence in all divisions of the 

 vegetable kingdom; only in particular families of the Algae, as, for 

 example, in the Palmellacese, Chlorococcacese, Oscilatorineae, and Nosto- 

 chineae, as also in the large-celled Cladopbora, and the unicellular Algae 

 with unlimited growth of the cell (Vaucberia, Codium, Caulerfa), no 

 trace of a nucleus has yet been discovered.' 



