216 THE BEGINNINGS OF LIFE. 



is capable (by virtue of its own inherent tendency to 

 undergo a process of differentiation) of taking on the 

 real cell form. Before a nucleus is evolved, whilst 

 still without a bounding membrane, the simple living 

 unit (plastide) is able to assimilate nutritive material 

 and grow ; it may be able to move from place to place 

 and continually vary in its form; it is able to divide 

 and reproduce its kind. In course of time a cell-wall 

 may consolidate around it, and a nucleus may arise in 

 its interior. The Cell is, therefore, seen to be only a 

 developed form, a more visibly complex condition, 

 which a simpler but already living and independent 

 Plastide may or may not assume. 



Some of the opinions we have just expressed were 

 uttered by Alexander Braun in 1851, when he said 1 : 

 c The cell is thus a little organism which forms its 

 covering outside, as the mussel, the snail, or the crab does 

 its shell. The contents enclosed by these envelopes form 

 the essential and original part of the cell, in fact must 

 be regarded as a cell, before the covering is acquired. 

 From the contents issues all the physiological activity 

 of the cell, while the membrane is a product deposited 

 outside, a secreted structure, which only passively shares 

 the life, forming the medium of intercourse between 

 the interior and the external world, at once separating 

 and combining the neighbouring cells, affording pro- 

 tection and solidity to the individual cell in connection 

 with the entire tissue. Hence the development of the 

 1 Loc. cit. p. 155. 



