66 THE REALISATION OF THE NUCLEAR FACTORS 



1952). In this way the nucleus may govern the main processes 

 of metabolism and construction in the cell. 



It is well known that the nucleus plays an important role in 

 cell division. It might be said that, at least in uni-nuclear cells, 

 each cell division is preceded by a nuclear division. Cell frag- 

 ments without a nucleus will not divide any more. This proves 

 that, generally speaking, there is a fixed causal relation between 

 nuclear and cellular division. This statement, however, can be 

 applied to the egg only with certain restrictions. In certain 

 cases, eggs that contain no nucleus at all are nevertheless able 

 to divide. E. B. Harvey (1938), for instance, treated enucleated 

 fragments of sea urchin eggs with parthenogenetic agents. This 

 led to cleavage, and even to developmental processes ("partheno- 

 genetic merogony") which went as far as to produce a more or 

 less normal looking blastula, consisting of cells without nuclei, 

 but then came to a stop. The same phenomenon was found in 

 amphibian eggs. This exception to the rule that non-nucleated 

 cells cannot divide can probably be explained as follows. During 

 the growth of the oocyte and after the disappearance of the 

 nuclear membrane of the germinal vesicle, a large quantity of 

 products of the nucleus has been accumulated in the cytoplasm 

 of those eggs. Divisions can go on in the absence of the nucleus, 

 therefore, until this store is consumed. It is significant, how- 

 ever, that in not even a single case has further differentiation 

 of non-nucleated cells been observed. 



The following observation also suggests that there is a 

 certain measure of autonomous activity in the cytoplasm during 

 cleavage. If the polar lobe of certain mollusc eggs is removed 

 at the first cleavage, rhythmic contractions and changes in 

 shape can be observed in the non-nucleated, isolated polar lobe ; 

 these movements coincide with the subsequent cleavages of the 

 rest of the egg. 



The correlation between the size of the cell and that of its 

 nucleus must also be mentioned here. This was studied, for 

 instance, by Boveri (1905) in sea urchin eggs. He compared 

 parthenogenetic or merogonic eggs (see p. 18 and 10), which 

 contained only half the normal number of chromosomes, with 

 normal eggs, and with eggs containing double the normal 



