76 S LECTUEES ON BIOLOGY 



difficult than it really is. There are two methods by which we 

 can gain this knowledge direct observation, and experiment. 



If, for instance, we observe under the microscope a plant-cell 

 about to strengthen its membrane in any one point by fresh 

 lamellae of cellulose it will be noticed that the nucleus leaves its 

 normal position in the centre of the cell and wanders to the point 

 under discussion. Here it remains until the work is completed, 

 when it returns to its original place. Does not this observation 

 suggest that the nucleus takes an active part in the construction 

 of the cell-wall ? Korschelt showed, further, that in the developing 

 ova of many animals the nucleus sends forth during the prepara- 

 tion of the food-yolk long pseudopodia-like shoots, and that the 

 production of yolk is most intensive in the direction of these 

 pseudopodia. It is therefore very probable that the nucleus 

 exercises also a certain influence upon the formation of yolk. 



Still more convincing are the replies which we obtain to 

 our questions from experiments ; minute though the protozoa are, 

 they are not so small that we cannot perform certain operations 

 with them. If we dissect the body of an Amoeba into a nucleated 

 and non-nucleated half the fate of the parts is very different. 

 At first the two animalcules thus created seem to have suffered 

 in nowise from this operation ; they round themselves off, pro- 

 trude their pseudopodia, crawl about, and may even be observed 

 to absorb food like healthy Amoebae. But whilst the nucleated 

 part digests and assimilates the nutriment the non-nucleated 

 part is unable to derive any advantage from its food which 

 remains undigested in the protoplasm. Soon we observe in the 

 non-nucleated part distinct signs of decay, and finally it disinte- 

 grates into a minute heap of granules. In the meantime the 

 other Amoeba has repaired the damage and grown once more to 

 normal size. Hence it is impossible to doubt that the protoplasm 

 without a nucleus cannot live, and that the nucleus plays the 

 leading part in metabolism, the nutrition of the cell. We 

 must, however, be careful to guard against an exaggeration of 

 the significance of the nucleus. As the protoplasm needs the 

 nucleus, so the nucleus needs the protoplasm. Verworn suc- 

 ceeded, by a clever operation, in taking the nucleus from a marine 

 protozoon, Thalassicola pelagica, a little ball of nearly 0'5 cm. in 

 diameter. This experiment proved that the nucleus invariably 



