NUCLEOLI 21 



and only gradually disappear. Similarly, on account of its density, 

 the nucleolus lies at the bottom of the large nucleus in an echinoderm 

 egg. When the egg is rotated the nucleolus appears to fall to the 

 bottom again (Gray, 1927). Presumably this is due to a movement 

 of the nucleolus in the nucleus and not to a movement of the nucleus 

 itself. 



In the Protista, nucleoli vary greatly in form, position and 

 behaviour (cf. Belar, 1926 h). In some species, nucleoli may lie on 

 the periphery of the nucleus [e.g., Amoeba terricola). In other 

 species they are numerous and rod-shaped, resembling chromosomes 

 (e.g., Euglypha). In others again an internal structure can be 

 distinguished (e.g., Chilodon). Nucleoli are not found in the small 

 compact nuclei of the generative cells of animals, but they are 

 sometimes found in those of plants (Upcott, 1936 b). As a rule they 

 increase in size with the resting nucleus. In the alga Acetabiilaria, 

 Hammerling (1932) found that the nucleus grew with the plant, 

 without any division, and as it grew the nucleoli fused and developed 

 into a single large branched body proportionate to the size of the 

 nucleus. 



The nucleoli are occasionally found to persist to metaphase of 

 mitosis. Sometimes this is clearly abnormal, as in the gametophyte 

 (poUen grain) of hybrid plants (D., 1929 c, in Tradescantia spp.). 

 Sometimes it is characteristic of a species (e.g., in Tulipa, Upcott, 

 unpublished ; Fritillaria, Frankel, 1936), and in the lower organisms 

 the nucleoli may be extruded into the cytoplasm and lost, or even 

 divide and pass into the daughter nuclei (Belar, 1926 b, Chlamy- 

 dophrys Schaudinni). In some species of Spirogyra nucleolar 

 behaviour is like that in higher plants. In others the chromosomes 

 become embedded in the nucleolus during the prophase and remain 

 so until telophase, the nucleolar mass dividing to the two poles. 

 In others again the nucleolus behaves in an intermediate way ; it 

 is shed by the chromosomes at the end of prophase and the chromo- 

 somes then become larger (Geitler, 1935 a and b). Nucleolar and 

 chromosome material thus seems to be spatially interchangeable. 

 In the higher plants, e.g., in Zea Mays, these bodies degenerate on 

 reaching the poles and do not enter the new nuclei (Yamaha and 

 Sinoto, 1925 ; Zirkle, 1928, 1931). Such bodies in Amoeba verrucosa 



