CELLS AND THE CELL THEORY 



47 



D 



ments act in totally different ways. Those that contain nu- 

 clear material are able to redevelop lost parts, to carry on their 

 life processes and to grow and multiply as usual; the fragments 

 that contain none of the nucleus, although they can move around 

 and apparently maintain life for a while, are unable to feed, or at 

 least to assimilate their food ; they are 

 unable to grow and unable to multiply; 

 Fig. 18. They have thus lost the most 

 essential features of life, since they 

 have lost the constructive power by 

 which protoplasm can assimilate and 

 grow. These experiments, repeated 

 many times over, show that the com- 

 plete life of a cell is impossible with- 

 out the presence of a certain amount 

 of nuclear material, but if nuclear 

 matter is present, the cell can carry 

 on its complete life, even though 

 the nucleus is itself cut into many 

 pieces. Such experiments, of course, 

 demonstrate very conclusively that 

 life functions cannot be carried on by 

 protoplasm alone, but only by proto- 

 plasm in combination with nuclear 

 substance. 



The Nucleus in Heredity. It is well to anticipate here one 

 further fact that demonstrates the great significance of the 

 nucleus. and chromatin. As we shall notice on a later page, 

 nearly all animals and plants show a form of reproduction in 

 which cells from two different individuals, male and female, 

 combine. This is known as sexual reproduction or fertilization. 

 When this union takes place, it is not the whole cells that com- 

 bine but only the nuclei; or still more accurately, it is the 

 chromatin material of the cells that combines rather than the 

 whole nuclei. The reconstructed cell contains chromatin ma- 



FIG. 18. STYLONYCHIA. 

 A SINGLE-CELLED ANIMAL 



If cut along the lines AB and 

 CD, only the middle piece con- 

 tains any nuclear matter; this 

 alone develops into a complete 

 individual, the other fragments 

 soon dying; n, the two nuclei. 



