6 INTRODUCTION 



chromosomes. These undergo a remarkable series of transforma- 

 tions, leading eventually to the segregation of the nuclear chromatin 

 in two separate daughter nuclei, each surrounded by a portion of 

 the original cytoplasm. Apart from its role in the division, and 

 therefore in the multiplication, of the cell, the nucleus is now known 

 to exert an influence perhaps not less important upon those chemical 

 changes in the cytoplasm which are necessary for its normal nutri- 

 tion and function.* It is doubtful whether any portion of proto- 

 plasm can permanently survive the loss of its nuclear material. It 

 must be remembered, however, that nuclear material may some- 

 times be present in diffuse form in cells which do not show a nucleus 

 in the histological sense. 



When we carry back the analysis of an organized body as far as 

 we can, we find that every organ of it is made up of cells, which 

 upon the whole conform to the type we have been describing, 

 although there are many differences in details. Some organisms 

 there are, low down in the scale, whose whole activity is confined 

 within the narrow limits of a single cell. The amoeba sets up in life 

 as a cell split off from its parent. It divides in its turn, and each 

 half is a complete amoeba. When we come a little higher than the 

 amoeba, we find organisms which consist of several cells, and 

 ' specialization of function ' begins to appear. Thus the hydra, the 

 ' common fresh- water polyp ' of our ponds and marshes, has an outer 

 set of cells, the ectoderm, and an inner set, the endoderm. Through 

 the superficial portions of the former it learns what is going on in 

 the world; by the contraction of their deeply placed processes it 

 shapes its life to its environment. As we mount in the animal 

 scale, specialization of structure and of function are found con- 

 tinually advancing, and the various kinds of cells are grouped 

 together into colonies or organs. In some organs and tissues the 

 bond of union is simple juxtaposition and similarity of function of 

 the constituent cells. But in others the union is protoplasmic, pro- 

 cesses of the cytoplasm actually passing from cell to cell. This is 

 seen in certain epithelial tissues, and conspicuously in the cardiac 

 muscle. 



The Functions of Living Matter. The peculiar functions of living 

 matter as exhibited in the animal body will form the subject of the 

 main portion of this book ; and we need only say here : (i) That in all 

 living organisms certain chemical changes go on, the sum total of 

 which constitutes the metabolism of the body. These may be 

 divided into (a) integrative or anabolic changes, by which complex 

 substances (including the living matter itself) are built up from 



* According to Hertwig, a precursor of chromatin, ' prochromatin,' a sub- 

 stance without characteristic staining reaction, is formed in the cytoplasm, 

 taken up by the nucleus, and there elaborated into chromatin. From the 

 nucleus chromatin and its derivatives return to the cytoplasm to be used in 

 its function. 



