258 THF MICROSCOPE. 



The successive changes in the cell contents furnish 

 other very important characteristics, such as the dis- 

 appearance and re-absorption of the nucleus ; this occurs 

 in every cell at some period of its existence ; in the cells 

 of the higher plants, the inner membrane, or primordial 

 utricle, entirely disappears. The Algae, and some few 

 unicellular plants, form an exception to the rule. In the 

 animal, the enlargement of the cell-wall takes place in a 

 uniform manner, whereas in the plant this is effected by 

 a deposition of successive layers on its inner surface, in the 

 shape of continuous rings, spiral bands, or other inter- 

 mediate forms. Then the wall not only increases in size, 

 but appears to possess a power of separating and appro- 

 priating certain substances, as lime, silica, lignine, &c., 

 which form the so-called cuticle. In animals as well as 

 in plants, new cells are formed within the old cells ; but 

 in the former, this process of a new formation begins in 

 the extracellular fluid, while in the latter it is mostly 

 endogenous. Multiplication of vegetable cells is effected 

 by three different modes : 1st, Many nuclei appear in the 

 maternal cell floating together with granular matter; 

 around each collects a minute vesicle, this gradually 

 increasing fills the maternal cell, which is eventually 

 absorbed. 2d, The internal substance of the cell divides 

 into two or more portions, each being furnished with a 

 nucleus. 3d, In the third mode of multiplication, the 

 wall itself of the maternal cell becomes gradually con- 

 stricted, and divides into two portions.* 



* "In most cells, especially when young, a minute, rounded, colourless 

 body may be seen, either.in the middle or on one side, called the nucleus. This is 

 very distinct in a cell of the pulp of an apple ; and within this nucleus is often 

 to be seen another smaller body, frequently appearing as a mere dot, called the 

 nucleolus. 



" The nucleus is imbedded in a soft substance, which fills up the entire cell ; 

 this 18 the protoplasm (protos, first, plasma, formative substance). As it is very 

 transparent, it is readily overlooked ; but it may usually be shown distinctly 

 by adding a little glycerine to the edge of the cover with a glass rod, when it 

 contracts and separates from the cell-walls. The protoplasm in some cells is 

 semi-solid, and of uniform consistence, while in others it is liquid in the centre, 

 the outer portion being somewhat firmer, and immediately in contact with the 

 cell-wall. In the latter case it forms an inner cell to the cell-wall, and is called 

 the primordial utricle. The terms ' protoplasm ' and ' primordial utricle ' are 

 however used by some authors synonymously. 



"The protoplasm is the essential portion of the cell, and it forms or secretes 

 the cell-wall upon its outer surface in the process of formation of the cell, con- 

 sidered as a whole. It is also of different chemical composition, from the cell 

 wall being allied in this respect to animal matter." Griffiths. 



