STRUCTURE OF ORGANISMS. 25 



the younger the organism the greater are its powers of re- 

 generation. Thus the embryo of a sea urchin, while still 

 composed of only a few cells, say two, four or eight, may 

 be divided into two, four or even eight separate cells, each 

 of which is capable of growing into a complete larva. 



All that is necessary for complete regeneration to take 

 place is protoplasm retaining its potentiality of develop- 

 ment, and present in sufficient quantity for metabolism to 

 be carried out in full. So soon as the potentiality 

 becomes limited by specialisation the power of regen- 

 eration becomes restricted. 



We must now return to the consideration of the repro- 

 duction of cells. A unicellular organism does not increase 

 indefinitely in bulk ; when it exceeds that size which is 

 normal for the adult of the species, it tends to divide into 

 two. The nucleus divides first, then the cell-body, form- 

 ing two daughter cells, each with its own nucleus. The 

 daughter cells then separate, growing into adults similar 

 to the original parent. It is the same with the cells of 

 the higher organisms. Each multicellular animal or 

 plant starts life as a single cell, which grows and divides 

 repeatedly. In every case the nucleus divides, each half 

 passing into one of the two daughter cells. But here, 

 instead of the cells separating to lead an independent life, 

 they remain more or less closely associated as parts of a 

 single complex individual (Fig. I). 



Thus not only is every cell derived from a pre-existing 

 cell, but every nucleus is formed from a pre-existing 

 nucleus, just as all protoplasm is derived from pre-existing 

 protoplasm. The continuity of protoplasm, of cells, and of 

 nuclei, is one of th6 most important facts established in 

 modern biology. 



Rarely the nucleus divides "directly," by simple con- 

 striction into two halves ; usually the division is "indirect," 

 by an elaborate process known as karyokinesis, and taking 

 place as follows. The chromatm gathers together into a 

 coiled thread, the linin network becomes disposed as a 



