THE REALISATION OF THE NUCLEAR FACTORS 65 



equivalent nuclei are formed, so that all the nuclei of the 

 organism (possibly with a few exceptions) will contain the full 

 complement of genes typical for the species. 



For this reason, Weismann's hypothesis that the whole of 

 development is governed by the nuclear factors cannot be 

 correct. On the contrary, nuclei and cytoplasm co-operate in 

 this process. We have discussed the complicated interplay of 

 the developmental processes in the cytoplasm. Evidently, the 

 nuclear factors must in some way or another exert their in- 

 fluence on these processes. 



It has been known for a long time that the nucleus is 

 generally indispensable for the life of the cell. Enucleated cell 

 fragments or protists may survive for a while but they show 

 no further development, and finally they die. In addition, the 

 nucleus is often to be found in those parts of the cell where 

 growth or differentiation takes place. This observation sug- 

 gests that the nucleus plays a leading, or at least an important, 

 role in these processes. Many investigators claim to have proved 

 that substances or particles produced by the nucleus pass into 

 the cytoplasm, either migrating through the nuclear membrane, 

 or mixing with the cytoplasm when this membrane has dis- 

 appeared during nuclear division (cf. what has been said above, 

 p. 58, about the ectoplasm of the ascidians). In many eggs the 

 nucleoplasm of the germinal vesicle has been shown to be rich 

 in sulphydryl compounds. These are poured out into the cyto- 

 plasm when the nuclear membrane disappears. It has also been 

 demonstrated that the nuclei of many cells (e.g. oocytes and 

 neurones) produce ribonucleic acid. This substance diffuses 

 through the nuclear membrane into the cytoplasm (Caspersson, 

 Hyden), where it may be used in the construction of microsomes 

 and mitochondria. The microsomes are probably concerned 

 mainly with protein synthesis (cf. p. 34), the mitochondria with 

 the cell's energy metabolism. The mitochondria are the site of 

 energy-supplying oxidation processes, as well as of phosphor- 

 ylations, which fix the liberated energy in the form of energy- 

 rich phosphate bonds, especially as adenosine triphosphate. 

 Substances produced by the nucleus are supposed to ensure 

 the coupling between oxidations and phosphorylations (Brachet, 



Raven - Outline Physiologie 5 



