102 CONTROL MECHANISMS IN CELLULAR PROCESSES 



called tlie secretion grannies cliromidia. At the time when most of 

 this research was done, it was not vet known that all cells have two 

 types of nncleic acids, and that the basophilia is mainly due to them. 

 It was not possible at that time to identifv the nuclear secretion 

 products as RNA, and they were sometimes confused with expelled 

 chromatic material (Goldschmidt, 1905). Only modern research 

 has shown that RNA is actually present in the granules secreted by 

 nuclei. 



It would be interesting to know if ribosomes are present in the 

 secretion product. Electron microscopy can provide a partial an- 

 swer to this problem, although no positive method of identifying a 

 ribosome in thin sections is available. Nucleoli, under the electron 

 microscope, show fine granular structure and these granules are in- 

 distinguishable from ribosomes (Gall, 1956; Bernhard, 1959). Simi- 

 lar granules are also found scattered in the nucleoplasm. Nuclear 

 secretion was studied by the electron microscope in several cells 

 (Anderson and Beams, 1956; Gay, 1956, De Groot et al, 1956). In 

 the pancreas, Clark ( 1960 ) could identify three types of secretion 

 material, one of them similar to the structure of nucleoplasm, an- 

 other similar to nucleoli. The secretion vesicles remained enclosed 

 in an evagination of nuclear membrane and disintegrated later. The 

 contents became scattered in the cytoplasm and the nucleolar type 

 of material could not be distinguished from nearby ribosomes. If all 

 the stages of nuclear secretion could be followed under the electron 

 microscope as carefully as was done before with the optical micro- 

 scope, new important information on nuclear function could be ob- 

 tained. 



Although nuclear secretion seems to be a property of a few types 

 of cells, it must be a general phenomenon, functioning as the trans- 

 fer of nuclear information into the cytoplasm. It can be observed 

 only in cells where large quantities of RNA are formed and liber- 

 ated. In most cells, perhaps only very small granules, unobservable 

 under the ordinary microscope, are continuously secreted. Individ- 

 ual ribosomes could leave the nuclei in this way. On the other hand, 

 it may be significant that it is the nucleolar material which leaves 

 the nucleus, as if ribosomes were first gathered in the nucleoli before 

 being released in the cytoplasm. 



Flies in the Ointment. The overwhelming evidence which has 

 been presented indicates that the nucleus and DNA are implicated 



