THE PHYSICAL BASIS OF HEREDITY. 



149 



plication, assimilation, secretion, excretion, irritability, 

 etc., functions which, in multicellular organisms, are di- 

 vided up among a vast number of the 

 Unicellular and constituent cells . Thus while the one- 



multicellular , ... 



organisms celled amoeba has its muscular, nervous, 



and digestive systems united within the 

 limits of a single microscopic mass of protoplasm, the 

 higher animals have their various functions divided up 

 among definite groups of thousands and millions of cells, 

 each group carrying out some particular function. In 

 response to this physiological division of labour among 

 the cells has come about a corresponding modification 

 in their structure, so that we find certain forms and types 

 characteristic of the particular function which the re- 

 spective cells carry out. The muscle cell, for example, 

 is one whose special work is that of contraction. Within 

 its substance has been developed a system of highly con- 

 tractile fibrils, and the whole cell has assumed an elon- 

 gated shape. For this one function of contractility have 

 been sacrificed more or less completely the other prop- 

 erties of protoplasm, and thus it has become dependent 

 upon its fellows which have assumed various other 

 functions. The bone cell, the gland cell, the epithelial 

 cell all have equally complicated specializations of 

 structure in other directions and, all united together 

 into an organic community, are co-ordinated and di- 

 rected in their various activities by the nerve cells. 



However diverse the form and function of the adult 

 tissues may be, they all have the same fundamental 



structure, and they all have a common 

 The essential od in and descent frQm th f ert ili ze d 



parts of the cell. 



egg cell. The essential parts of a cell 



consist of the cell body and the cell nucleus, which to- 

 gether make up the living substance. The body of the 

 cell is made up principally of a granular, viscid, semi- 



