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 
constituent cells. Thus while the one- 
celled amceba 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 = Grigin and descent from the fertilized 
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- 
Unicellular and 
multicellular 
organisms, 
