CELL FORMATION. 



529 



The wall of a cell is chemically different from the nu- 

 cleus; if treated with dilute nitric acid, the wall is dis- 

 solved, and the nucleus unaffected, so that we can in this 

 manner isolate the nucleus. It is not known whether 

 there exists any chemical difference between the latter and 

 the nucleolus; and it is probable that the nucleolus is a 

 space in the nucleus containing a fluid. 



The elementary cell is imbedded in an amorphous 

 matter, which is termed cytoblastema, and is a fluid of 

 greater or less consistence; so that in one case the cell 

 may float, and in the other it may be imbedded. The 

 matter between cells is called intercellular substance. Cells 

 differ in their contents, 

 which implies a difference 

 in their walls, inasmuch as 

 they secrete the interior. 



The nucleolus was first 

 discovered by Robert 

 Brown in plants, and its 

 use made out by Schlei- 

 den. He discovered that 

 the nucleus was formed 

 before the cell, and the 

 latter was formed around 

 it. After this, cells of 

 different kinds were found Fig. 270. Development of viva. 



in animals, and Schwann A, isolated cells resembling those of some 



Vertebrata. E and c, clustering of the 

 same. D, cells in the filamentous stage 



collected many instances, 



showing that animal and (confervoid). 



vegetable tissues were developed from cells : this was an 



important generalisation* 



The mode of origin of the nucleated cell is this : cyto- 

 blastema first exists ; in it is developed the nucleus, and 

 around the latter is formed the membrane of the cell from 

 matters drawn from the cytoblastema. The way in which 

 the cell itself is formed is this : around a granule we have 

 a deposit forming a nucleolus, round it again is formed 

 the nucleus; a swelling of this membrane takes place ; at 

 length it covers over all like a watch-glass, and thus 

 the exterior cell is formed. (Fig. 269, No. 1.) Coalescence of 

 the grannies forms a nucleus, and in the interstices be- 

 M M 



