132 



STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



onic life (CHAP. XT.) The process of subdivision seems to commence in 

 the nucleus, which tends to separate itself into two equal parts ; and 

 each of these draws around it a portion of the contents of the cell, so 

 that the cell-wall, which is at first merely inflected inwards by a sort of 

 hour-glass contraction, at last forms a complete partition between the 

 two halves of the original cavity (Fig. 17, A-D). The process of subdi- 

 vision may be again repeated, either in the same or in a contrary direc- 

 tion, so as to produce four cells, either linearly arranged (F, G, H), or 

 clustered together (E) ; and this duplication may take place until a large 

 mass has been produced by the subdivision of a single original cell. In 



Fig. 17. 



Multiplication of Cartilage-cells by duplication: A, original cell ; B, the same beginning to divide; c, the 

 same showing complete division of the nucleus ; D, the same with the halves of the nucleus separated, and 

 the cavity of the cell subdivided; E, continuation of the same process, with cleavage in contrary direction, to 

 form aclueter of four cells; F, G, H, production of a longitudinal series of ceils, by continuation of cleavage 

 in the same direction. 



other cases, however, the nucleus appears to break up at once into several 

 fragments, each of which may draw around it a portion of the contents 

 of the parent-cell, which becomes invested by a cell-wall of its own ; and 

 thus the cavity of the parent-cell may at once become filled with a whole 

 brood of young cells, without any successive subdivision. Of this pro- 

 cess we frequently have examples in the case of morbid growths, in which 

 the multiplication of cells often takes place with great rapidity (Fig. 18), 



Fig. 18. 



Parent-cells, a, a, of cancerous structure, containing secondary cells, b, b, each having one, 

 two, or three, nuclei, c, c. 



Generally. speaking, the former method seems to prevail in structures 

 which have a comparatively permanent destination ; whilst the latter is 

 adopted in cases in which the life of the cells thus generated is but transi- 



