CONTENTS OF THE CELL 535 



it retains its freedom to wander about, the course of the -treams 

 alters in conformity with its position. /But it is in the multiplication 

 of cells by binary subdivision, which will be presently described, that 

 the speciality of the nucleus as the centre of the ,-itfil m-tir'dy of the 

 cell is most strongly manifested. The chlorophyll cr>rj,sdes, which 

 are limited to the cells of the parts of plants acted on by light, are 

 specialised particles of protoplasm through which a green colouring 

 matter is diffused ; and it is by them that the work of decomposing 

 CO 2 , and of* fixing ' its carlxm by union with the oxygen and hydrogen 

 of water into starch, is effected. The characteristic green of 

 chlorophyll often gives place to other colours, which seem to be pro- 

 duced from it by chemical action. , Starch grains are always formed 

 in the first instance in the interior T>f the chlorophyll corpuscles and 

 gradually increase in size until they take the places of the corpuscles 

 that produced them. So long as they continue to grow, they are 

 always imbedded in the protoplasm of the cell ; and it is only when 

 fully formed that they lie free within its cavity. 



But although these component parts may be made out without 

 any difficulty in a large proportion of vegetable cells, yet they cannot 

 be distinguished in some of those humble organisms which are 

 nearest to the border-line between the two kingdoms. For in them 

 we find the ' cell-wall ' very imperfectly differentiated from the ' cell- 

 contents ; ' the former not having by any means the firmness of a 

 perfect membrane, and the latter not possessing the liquidity which 

 elsewhere characterises them. And in some instances the cell is 

 represented only by a mass of endoplasm, so viscid as to retain its 

 external form without any limiting membrane, though the superficial 

 layer seems to have a firmer consistence than the interior substances ; 

 and this may or may not be surrounded by a gelatinous-looking 

 envelope, which is equally far from possessing a membranous firmness, 

 and yet is the only representative of the cellulose wall. This viscid 

 endoplasm consi>t>. as elsewhere, of a colourless protoplasm, through 

 which minute colouring particles may be diffused, sometimes uni- 

 formly, sometimes in local aggregations, leaving parts of the proto- 

 plasm uncoloured. The superficial layer in particular is frequently 

 destitute of colour ; and the partial solidification of its surface gives 

 it the character of an i ectoplasm.' Such individualised masses of 

 protoplasm, destitute of a true cell-wall, have sometimes been 

 termed * primordial cells.' It is an extremely curious feature in 

 the cell-life of certain protophytes that they not only move like 

 animalcules by cilia or flagella, but that they exhibit the rhythmically 

 contracting vacuoles which are specially characteristic of protozoic 

 organisms. 



So far as we yet know, every vegetable cell derives its existence 

 from a pre-existing cell : and this derivation may take place (in the 

 ordinary process of growth and extension, as distinguished from 

 -xual" multiplication') in one of two modes: either (1) binary 

 "h(lh'ision of the parent-cell, or (2) free-cell formation within the 

 parent-cell. The first stage of the former process consists in the 

 elongation and transverse constriction of the nucleus; and this con- 

 striction becomes deeper and deeper, until the nucleus divides itself 



