202 THE STUDY OF ANIMAL LIFE CHAP, xi 



it can conveniently grow, it divides ; in other words, it 

 reproduces at the limit of growth, when processes of 

 waste are gaining on those of repair. By dividing, the 

 mass is lessened, the surface increased, the life continued. 



But although we thus get a general rationale of cell- 

 division, we are not nearer a conception of the internal 

 forces which operate when a cell divides. In most cases 

 the process is orderly and complex, and is somehow 

 governed by the behaviour of the nucleus. Few results 

 of the modern study of minute structure are more mar- 

 vellous than those which relate to dividing cells. From 

 Protozoa to man, and also in plants, the process is in its 

 essentials strikingly uniform. The nucleus of the cell 

 loses its membrane ; the centrosome divides into two 

 and one lies at each pole ; the coil of chromatin threads 

 becomes resolved into a definite number of rod-like 

 chromosomes ; these become arranged on the equatorial 

 plane and each is longitudinally cleft in two ; one half 

 of each goes to one pole, the other half to the other ; the 

 cell is divided across the equator ; and there is a recon- 

 struction of the daughter-nuclei in the two cells which 

 thus arise from one. The division is most thorough, 

 each of the two daughter-cells getting an accurate half 

 of the original nucleus. (See Fig. 54.) 



We study the nucleus, first as a simple kernel which 

 divides, years afterwards as composed of a network or 

 coil of nuclear threads ; later still as a focus of chromo- 

 somes which seem ever to become more marvellous, 

 " behaving like little organisms." After all our analysis, 

 both chemical and physical, we are baffled by the move- 

 ments of the protoplasm, the cycle of phases in the 

 nucleus, the behaviour of the centrosomes, the man- 

 oeuvres of the chromosomes ; indeed in regard to the 

 whole give and take, combination and opposition, among 

 the many members of the " cell-firm," as we may well 

 call it, we must admit that they behave as they do 

 because they are alive. 



From the cell as a unit element we pass in our struc- 

 tural analysis to the protoplasm itself, the physical basis 

 of life ; and the important general fact is simply that 



