THE STRUCTURAL BASIS OF THE BODY 33 



cell containing a nucleus by means of fine threads of protoplasm 

 which pass through pores in the intercellular septa (Fig. 10). In the 

 higher animals we have, in the case of the nerve-cell, an example of 

 the necessity of the nucleus for growth. Here division of the nerve 

 fibre causes degeneration of the whole fibre separated from the cell 

 containing the nucleus, and regeneration of the fibre, when it occurs, 

 is effected by a down -growth of that part of the fibre which is still in 

 connection with the nucleus. All these facts show that the power of 

 morphological as well as of chemical synthesis depends on the presence 

 of a nucleus. On this account the nucleus, as we shall learn later on, 

 must be regarded as the especial organ of inheritance. The trans- 

 mission of the paternal qualities from one generation to the next is 

 effected by the entrance simply of the nuclear material of the male cell, 

 the spermatozoon, into the ovum. In the words of Claude Bernard, 

 " the functional phenomena in which there is expenditure of energy 

 have their seat in the protoplasm of the cell (i.e. the cytoplasm). The 

 nucleus is an apparatus for organic synthesis, an instrument of produc- 

 tion, the germ of the cell." 



Similar conclusions may be drawn from a study of the changes in 

 the nucleus which accompany different phases in the activity of the 

 whole cell. 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 FIG. ll. Branched nucleus from 



1 , , . , , . the spinning gland of butterfly 



the outgrowth always takes place in i arva (pi er is). (KOESCHELT.) 



the immediate neighbourhood of the 



nucleus, which is carried forward and remains near the tip of the 

 growing hair. The active growth of cytoplasm, 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 which 

 interchanges can go on between 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 



3 



