22 LIFK : OUTLINES 01- GENERAL BIOLOGY 



. . . It is this power of spontaneous division which most sharply 

 distinguishes the living from the non-living. . . . The greatest 

 advance I can conceive in biologj' would be the discovery of the 

 instability which leads to the continued division of the cell. When 

 I look at a dividing cell I feel as an astronomer might do if he 

 beheld the formation of a double star: that an original act of 

 creation is taking place Ix'fore me." 



In the present youthful condition of biology it is wise to return 

 at frequent intervals to concrete illustrations. We need the warmth 

 of actual facts to help us to aj^preciate the cpiality of reproductivity 

 which we are only lx*ginning to understand. In one day the multi- 

 plication of a micrcjbe may result in a number with thirty figures. 

 Were there an annual plant with only two seeds, it could be repre- 

 sented by over a million in the twenty-first year. But a common 

 British wred {Sisytuhrittm officinale) has often three-quarters of a 

 million of seeds, so that in three years it could theoretically cover 

 the whole earth. Huxley calculated that if the descendants of a 

 single green-lly all survived and multij)lied, they would, at the end 

 of the first summer, weigh down the poi)ulation of China, A codfish 

 is said to pnxluce two million eggs, a conger eel ten millions, an 

 oyster twenty millions. The starfish Luidia, according to Mortensen, 

 produces two hundred million eggs every year of its life. 



I)i:vi:l()I'mi:nt. — In active tissues, like muscle or^gland, wear 

 and tear is inevitable, esjx'cially in the less labile parts of the cells 

 —the furnishings of life's laboratories, such as the for the most 

 part ultra microscopic films that partition the cytoplasm into areas. 

 When the results of the wear and tear over-accumulate, they tend 

 to depress activity and in time to inhibit it; and this means ageing, 

 towards (hath. But this decline of vitality may be counteracted 

 by rejuvenescence-processes in the ageing cells, or by the replace- 

 ment <»f worn-out cells by new ones. In some cases the hard-worked 

 cells go fatally out of gear, as in the brain of the busy summer-bee, 

 which d(H\s not usually survive for more than six or eight weeks. 

 In other cas<s, as in ordinary muscle, the recuperation afforded by 

 fcKxl and rest is very jxrfect, and the same cell may continue active 

 for many years. Such c<lls are comjiarable to the relatively simple 

 unicellular animals, like the anifiba-, which recuperate so thoroughly 

 that th« y evade natural (hath altogether. In another set of cases, 

 e.g. the linmg c( lis of the stomach, or the epithelium covering the 

 lil>s, the senescent cells die and drop off, but are replaced by others. 

 The t)uter epidermic layer of the skin (the stratum corneum) is 

 continually wearing aw.»y, and as continually being replaced by 

 contributif)ns from the more intensely living and growing deeper 

 stratum (the stratum Malj>ighii). Similarly at the tip of a rootlet 

 there is a cap of cells which are always dying away and being 

 replaced from the delic ate growing jx>int which they protect. I'rom 



