NUCLEOLUS 187 



Zirkle claims to have separated nucleolar material from the 

 chromatin of the reticulum and also from mitochondria in Zea 

 mays. In the resting nucleus all the chromatin is localised in the 

 reticulum and the nucleolus contains none. It was found impos- 

 sible to detect whether any strands connect the nucleolus with 

 the reticulum in the living resting nucleus. At prophase of 

 mitosis, however, a very distinct connection can be seen to be 

 established between the nucleolus and spireme and this connection 

 is attached to the narrow end of the rather pear-shaped nucleolus. 

 Often a double connection can be detected owing to the splitting 

 of the nucleolus and nucleolar material can be seen flowing into 

 the spireme across the connecting bridge. 



It was found that when the root tips were fixed in zinc or 

 nickel bichromate, results somewhat contradictory to this view 

 were obtained. But when the material was fixed in nickel- 

 chrome-acetate (2?H 5-0 — 5-2) the entrance of nucleolar material 

 into the developing spireme could be very readily seen, as this 

 fixative dissolves all the chromatin and mitochondria. iVt a later 

 stage the nucleolus can be seen connected at two places with the 

 spireme ; it then becomes drawn out and finally constricted into 

 two parts which pass to opposite poles of the spindle. The 

 nucleolar material thus enters the daughter nuclei in two ways : 

 (1) in the chromosomes, and (2) as a distinct body. The latter 

 reaches the poles of the spindle first, and the nucleolar globules 

 then proceed to fragment, the majority of the fragments 

 passing into the cytoplasm, while the chromosomes are still on the 

 equatorial plate. At telophase the nucleolar material which has 

 been contained in the chromosomes collects into droplets, which 

 flow together and reform the nucleolus of the resting nucleus once 

 more. The nucleolar material is thus continuous, and is derived 

 from pre-existing nucleolar material ; a certain amount, however, 

 is lost at each division by the process of fragmentation and ulti- 

 mate disappearance described above. Zirkle considered that the 

 nucleolus has two possible functions : (1) that it transmits the 

 influence of the genes to the organism, and (2) by means of its 

 electro-positive charge it serves as a framework for the distribu- 

 tion of chromatin to the daughter cells. 



