MAN FROM THE FARTHEST PAST 
excretion of worn-out or waste substances; sensitivity, com- 
prising the reception and transmission of stimuli, so that 
what is done to one cell produces some sort of effect in 
others; and reproduction by cleavage, so that one cell 
becomes two. Although reproduction by cleavage is the 
property of all living cells and is necessary for the growth 
and repair of tissues, certain cells in the higher forms of 
life are reproductive cells par excellence, since they have the 
function of producing new individuals. 
Cells contain, besides the microscopic nucleus, the nour- 
ishing and specialized material called cytoplasm, con- 
tained within a sur- 
rounding membrane. 
Imbedded in the cyto- 
plasm, the nucleus 
itself is also inclosed 
une : most of the time 
Fea I, within an inner mem- 
Lo ‘ ; brane, which contains 
a fluid called nuclear 
sap and also the all- 
important chromo- 
somes of the nucleus 
wherein is the seat of 
inheritance units. The 
cell goes through two 
phases, which may be 
compared to sleeping 
and waking. In the 
dormant state of the 
cell the chromosomal 
structure of the nu- 
cleus is practically in- 
visible, though un- 
doubtedly this is only 
one, 
iA : 
Fic. 2. Phases of mitosis or cell multiplica- 
tion by division. After Schafer, from Prentiss an apparent absence, 
and Arey not arealone. Inthe 
[ 26 | 
