36 



GENERAL CONCEPTS 



channels through the nuclear membrane through which the nucleoplasm 

 and cytoplasm "are continuous. The nucleus is required for growth and 

 for cell division, but some cells, the ameba, for example, can survive 

 for many days after the nucleus has been removed by a microsurgical 

 operation, 'l^o demonstrate that it is the absence of the nucleus, not the 

 operation itself, that causes the ensuing death, one can perform a sham 

 operation. A microneedle is inserted into an ameba and moved around 

 inside the cell to simulate the operation of removing the nucleus, but 

 the needle is withdrawn without actually removing the nucleus. An 

 ameba subjected to this sham operation will recover, grow and divide. 

 A controlled experiment such as this, in which two amebas are subjected 

 to the same operative trauma and the one with the nucleus lives whereas 

 the one without the nucleus dies, provides strong evidence of the vital 

 role of the nucleus in the metabolic processes that underlie growth and 

 cell division. 



A classic demonstration of the role of the nucleus in the control of 



Figure 3.3. Hammerling's demonstration of the production of an umbrella-regener- 

 ating substance by the nucleus of Acctabularia. See text for discussion. (Villee: Biology.) 



