56 ORGAXOGRAPHT. 



by any one by the application of iodine and dilute sulphuric acid 

 tosuch tissues (^(/.s. 137 and 138). The existence of this intercel- 

 lular substance has been even denied altogether, but by far the 

 majority of observers agree as to its presence, to some extent at 

 least, in plants. Further observations are however needed be- 

 fore we can arrive at any definite conclusions respecting it. 



Section 3. Formation and Growth of Cells. 



The subject of cell-formation or cytogenesis, has of late years 

 engaged the attention of many able physiologists, by whose united 

 labours we have now arrived at tolei'ably definite conclusions upon 

 the main pointsof the inquiry, although many of the subordinate 

 ones are still involved in obscurity. Our limits will not allow 

 of describing in detail all the theories of cell-formation which 

 have been from time to time brought forward by different 

 observers, neither is such necessary, since all are now agreed 

 upon the essential principles of the process; but we shall confine 

 our attention to a general outline of the subject. 



Cells can only be formed from a thickened fluid which is 

 contained in the interior of cells, or has been previously elabo- 

 rated by their agency; hence cells can in no case be formed 

 without the influence of living organisms. The fluid which 

 thus forms the pabulum for the formation of cells, and which 

 must necessarily contain all the materials which ultimately enter 

 into the composition of those cells, has been appropriately 

 termed protoplasm, the nature of which we have already fully 

 described. By other observers this formative matter of cel^ 

 has been variously called organizable matter, vegetable mucilage, 

 cytoblastema, &c. 



All cells originate in one of two ways: either free in the 

 cavities of older cells, or at least in the protoplasmic fluid 

 elaborated by their agency; or by the division of such cells. The 

 first is called Free Cell-formation, the second Cell-division. 



1. Free Cell-formation. — Wedistingui.'»h two modifications 

 oi free cell-formation. 1. Free cell- formation from a nucleus or 

 cytoblast, and 2. Without the previous formation of a nucleus. 



1. Free Cell-fonvation from a nucleus. — This mode was first 

 discovered by Schleidcn, who considered it to be the only ])ro- 

 cess of cell-formation occurring in plants. Subsequently he 

 modified his views materially, not only as regarded the manner 

 in which it took ])lace, hut also as to its universality, and he 

 now admits that it is only one ])rincipal mode of cell-formation. 

 The man!icr in which it is su])i)oscd to take place is as follows 

 {figs. 139 and 140): — a ])ortion of the jirotojilasm collects 

 into a more or less rounded or somewhat oval form, with a 

 defined outer border, thus lormingthe nucleus of the cell; upon 

 this a layer of jirotoplasni is dcixtsitcd, which assumes the form of 

 a meml)rane, and expands so as to form a vesicle; on the outside 

 of this a cellulose njcnibrane is secreted, and the fornmtioD of 



