CELLS; THEIR MODE OF MULTIPLICATION. 43 



to its wall, there is usually to be observed a solid collection 

 of granular matter, which is termed the nucleus (fig. 4, a a). 

 The typical form of the cell is globular or oval (fig. 5) ; but 

 when a number of cells are in contact with each other, and 

 are pressed together, their sides become . flattened ; so that 

 when they are cut across no intervals are seen between them, 

 but their walls are everywhere in contact (fig. 6), just as in 



Fig. 5. Fig. 6. 



ROUNDED CELLS IN CARTILAGE POLYGONAL CELLS FROM CAR- 



OF BAT'S EAR. TILAGE IN MOUSE'S EAR. 



the section of a vegetable pith. The chemical composition of 

 the nucleus differs from that of the cell-wall ; for whilst the 

 latter is dissolved by acetic acid, the former (like the yellow 

 elastic tissue, with which its substance appears to have some 

 relationship) is unchanged by it. "When the formation of a 

 cell is complete, and it is not destined to reproduce its kind, 

 the nucleus frequently disappears ; this is the case, for 

 example, with the red corpuscles of the blood of Mammalia 

 ( 229), and also with Fat-cells ( 46). 



33. New cells may originate in one of two very distinct 

 modes ; either from a pre-existing cell, or by an entirely new 

 production in the midst of an organizable fluid or blastema. 

 The most remarkable example of the first process is presented 

 in the early development of the germ, which entirely consists 

 of an aggregation of cells, every one of which undergoes 

 successive subdivisions into two, so that the total number 

 in the germ-mass is repeatedly doubled (Chap. xv.). The 

 same method of multiplication by binary subdivision may be 

 seen to continue throughout life in Cartilage-cells ( 47), the 

 growth of which almost exactly repeats the history of the 

 growth of the lowest forms of Sea-weeds. The process of sub- 

 division seems to commence in the nucleus, which begins to 

 separate itself into two equal parts, and each of these draws 



