28 



FUNDAMENTALS OF CYTOLOGY 



Avithout visible injury. The relative specific gravities of the nuclear com- 

 ponents and other portions of the protoplast can be determined by the use 

 of the centrifuge (Fig. 30). In an electrical field, free nuclei or cells very 

 rich in nuclear material tend to pass toward the anode, showing that they 

 carrj^ a negative charge, whereas cytoplasm or cells with little chromatin 

 tend to go in the opposite direction. In the nucleus it is the chromatin 

 that carries the negative charge, the karyolymph and usually the nucle- 

 olus being positive. 



The nucleus, like the rest of the protoplast, consists of chemical sub- 

 stances of several classes, among which proteins, lipides, and water play 

 the major roles. Of special importance is the chemical nature of the 

 chromatic threads and the chromosomes of which the}^ are the principal 

 constituents, for it is largely upon their composition that the peculiar 

 powers of the nucleus in determining the course of development and the 



Fig. 17. — Three stages in mitosis in einl)ryo of Ascaris, showing centrioles and asters. 



phenomena of heredity seem to depend. Anal3^ses have shown that the 

 chromatin is a nucleoprotein composed of nucleic acid and certain basic 

 proteins. Further discussion of the chemical nature of nuclei w\\\ 

 follow in the next chapter (page 51). 



The Centrosome.— In the cytoplasm of most animal cells and of 

 certain lower plant cells a centrosome is commonly present. It is not 

 found in seed plants. Typically it consists of a minute, deeply staining 

 granule, the centriole, or often a pair of these, surrounded bj^ a mass of 

 less stainable substance, the centrosphere. In some cells only a centriole 

 is visible, while in others nothing but centrosphere substance seems to be 

 present. The aspect of the centrosome varies widely in cells of different 

 kinds and especially in different stages of nuclear division, when one of its 

 chief functions is evidently performed (Fig. 17). It also plays a con- 

 spicuous role dining the development of certain motile cells, notably male 

 gametes, where it is concerned in the formation of the motor apparatus. 

 Further discussion of centrosomes is therefore deferred to chapters dealing 

 with these topics. 



