62 INTRODUCTION TO CYTOLOGY 



such active movement can be directly observed in the nucleus of the 

 living cycad spermatozoid. In the long, narrow cells of vascular bundles 

 the nuclei, which are not free to grow in all dimensions, come to be 

 correspondingly elongated. The nucleus may also be passively forced 

 into very irregular shapes by the dense accumulation of starch grains 

 and the diminution in the amount of cytoplasm, as in the endosperm 

 cells of maize. In Stentor and Spirostomum the nucleus has the form 

 of a string of beads (Fig. 17, B). 



In size the nucleus shows a wide variation, ranging in plants from 

 the extremely minute nucleus of Mucor, IM or less in diameter, to the 

 relatively gigantic nucleus of the Dioon egg, with a diameter of 600 AJ. 

 A similar range is seen in animal nuclei. Although the nuclei of the 

 fungi are characterized by small size, most of them being less than OM 

 in diameter, they may grow to a large size at certain stages. The primary 

 nucleus of Synchytrium, for instance, reaches a diameter of over 60M- 

 The majority of nuclei, however, fall between 5n and 25^- In spite of 

 the wide range in the size of nuclei of different organisms, in a given 

 tissue it is comparatively uniform. 



With respect to the physical nature of the nucleus as a whole, the 

 researches of Kite (1913) and Chambers (1914, 1917) have shown that it 

 ordinarily consists at least in part of a gel of higher viscosity than the 

 cytoplasm, often being so firm that it can easily be handled without 

 injury by means of the dissecting instrument. This obviously would be 

 impossible were the nucleus merely a watery droplet or vesicle in the cyto- 

 plasm. The germinal vesicle (nucleus) of the animal egg Chambers 

 (1917) finds to be a sol droplet with a gel membrane; if it is pinched in two 

 by the dissecting instrument the two halves will reunite if they come in 

 contact. 



The chemical nature of the nucleus has been dealt with in the preced- 

 ing chapter. With regard to its electrical properties, the nucleus is 

 apparently negative to the cytoplasm. R. S. Lillie (1903) found that 

 free nuclei and the heads of spermatozoa, which are almost entirely 

 nuclear material, pass to the anode in an isotonic cane sugar solution, 

 whereas cells rich in cytoplasm, such as large leucocytes, pass to the 

 cathode. These results have been confirmed by Hardy (1913). 



Nucleoplasmic Ratio. Of more importance than the absolute size 

 of the nucleus is the relation of its volume to that of the cytoplasm the 

 so-called nudeoplasmic or Kernplasma relation. Many years ago it was 

 held by Sachs (1892, 1893, 1895) and by Strasburger (1893) that the size 

 of a meristematic cell in a plant, owing to a supposed limitation of the 

 sphere of influence of the nucleus, maintains a very definite relation to 

 the size of its nucleus. This conception has recently been emphasized 

 anew by Winkler (1916), and parallel views have been expressed by 

 several zoologists (e.g., Hegner on Arcella, 1919). In the case of certain 



