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. 
Rovunpep CELLs 1x CARTILAGE PotycoraL CELts FRom CaR- 
or Bat’s Ear. TILAGE 1s Movsez’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 fiuid 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 
i im 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 
parate itself into two equal parts, and each of these draws 
