144 THE BEGINNINGS OF LIFE. 



which the vital manifestations of the organism as a 

 whole depend. On this subject much difference of 

 opinion exists. Though we cannot go into detail, 

 we will briefly consider the doctrines which have been 

 principally advocated. 



An enormous impulse was given to such enquiries by 

 the publication, in the year 1839, of the researches of 

 Schleiden and Schwann 1 , who endeavoured to prove 

 that all the tissues of both plants and animals were 

 entirely built up of morphological units called c cells.' 

 They believed that cells were continually being produced 

 de novo in the bodies of plants and animals. Speaking 

 on this subject Schwann said 2 : c The following admits 

 of universal application to the formation of cells; there 

 is in the first instance a structureless substance present, 

 which is sometimes quite fluid,, at others more or less 

 gelatinous. This substance possesses within itself, in 

 a greater or less measure, according to its chemical 

 qualities and the degree of its vitality, a capacity to 

 occasion the production of cells. When this takes 

 place the nucleus usually appears to be formed first, and 

 then the cell around it. The formation of cells hears the 

 same relation to organic nature that crystallisation does to in- 

 organic. The cell when once formed continues to grow 

 by its own individual powers, but is at the same time 

 directed by the influence of the entire organism, in such 



1 ' Microsc. Researches into the Accordance in the Structure and 

 Growth of Animals and Plants.' Translation (Sydenham Society), 1847. 



2 Loc. cit. p. 39. 



