THE STRUCTURAL BASIS OF THE BODY 31 



Thus in growing plant-cells the nucleus is always situated at the point of 

 most rapid growth. In the formation of epidermal cells the nucleus moves 

 towards the outer wall and remains closely applied to it so long as it is growing 

 in thickness. When this growth is finished the nucleus moves to another 

 part of the cell. In the formation of root hairs the outgrowth always takes 

 place in the immediate neighbourhood of the nucleus, which is carried forward 

 and remains near the tip of the growing hair. The active growth of cyto- 

 plasm, which accompanies the activity of 

 secreting cells, is always associated with 

 changes in the position and in the size of 

 the nucleus. Where the nutritive activity 

 of the cell is very intense, as in the silk 

 glands of various lepidopterous larvae, the 

 nucleus is found to be very large and much 

 branched (Fig. 11) so as to present the 

 greatest possible extent of surface through Flo 1L Branohfld ' nucleu8 from 

 which interchanges can go on between the spinning gland of butterfly 

 nucleus and cytoplasm. 



The important changes which the nucleus undergoes in the process 

 of cell division we shall have to consider more fully in the later chapters 

 of this work. In the function of assimilation it is natural to assume that 

 it is those constituents of the nucleus which are peculiar to it both morpho- 

 logically and chemically, namely, the chromatin filaments, which are most 

 directly concerned. This assumption receives support from the changes 

 which have been observed to occur in these filaments during various phases 

 of nutritive activity of the cell. The staining powers of chromatin are in 

 direct proportion to the amount of nuclein it contains. In the eggs of the 

 shark it has been shown that the chromosomes undergo characteristic changes 

 during the entire growing period of the egg. At first they are small and stain 

 deeply with ordinary nuclear dyes, but during the period of growth they 

 undergo a great increase in size and at the same time lose their staining 

 capacity,* their surface being increased by the development of long threads 

 which grow out in every direction from the central axis. As the egg 

 approaches its full size, the chromosomes diminish in size and are finally 

 reduced to minute intensely staining bodies which take part in the first 

 division of the egg preparatory to its fertilisation (Fig. 12). We must 

 conclude that whereas the processes -of destructive metabolism or dissimila- 

 tion, which determine the activity of the cell, have their immediate seat in 

 the cytoplasm, the processes of constructive metabolism which lead to the 

 formation of new material, to the chemical and morphological building up of 

 the cell, are carried out in or by the intermediation of the nucleus. 



HISTOLOGICAL DIFFERENTIATION OF CELLS. Even within the 



limits of a single cell, differentiation of structure can take place by the 



setting apart of distinct portions of the cell for isolated functions. Thus in 



an organism such as vorticella the cell ia shaped somewhat like a wine-glass, 



* Riickerfc, cited by Wilson. 



