130 



THE PHILOSOPHY OF BIOLOGY 



now known as the blastula, can swim about by the 

 movements of these cilia. Further development results 

 in another larval form — the gastrula, and yet another, 

 the pluteus larva. After this the transformation into 

 the fully formed sea-urchin occurs. 



With various modifications this scheme represents 

 the early development of a very large number of animals 

 belonging to most groups. 



If we study the process of cell-division we shall 

 find it very complicated. The ovum, immediately 

 after fertilisation, consists of two main parts, the 

 nucleus and the cytoplasm. 



Within the nucleus is 

 a substance distinguishable 

 from the rest ; it is distri- 

 buted in granules and is called 

 the chromatin (i). When the 

 cell is about to divide this 

 chromatin becomes arranged 

 in a long coiled thread (2), and 

 then (3) this chromatic thread 

 breaks into short rods called chromosomes. Two little 

 granules now appear, one at each end of the nucleus, and 

 very delicate threads, the asters, appear to pass from each 

 of these bodies towards the chromosomes (4). Each 

 of the latter then splits lengthways into two, and a 

 half chromosome appears to be drawn by the asters 

 towards the poles of the nucleus. The latter then 

 divides (5) and then the whole cell divides. What 

 thus, in essence, happens in nuclear divisions is that 

 the chromatin of the nucleus is more or less accurately 

 halved. Apparently this substance consists of very 

 minute granules and the whole process is directed 

 towards the splitting of each of these granules into two. 

 A half-granule then goes to each of the daughter nuclei. 



Fig. 13. 



