2 GERM-CELL CYCLE IN ANIMALS 
rudiments already preformed. This belief could not 
continue to exist after Caspar Wolff’s brilliant 
researches proved that adult structures arise grad- 
ually from apparently undifferentiated material ; that 
is, development is epigenetic. Epigenesis, however, 
does not explain development; it simply maintains 
that it occurs. 
During the years since the theory of epigenesis 
was proposed a new theory of preformation has 
entered into our conception of development, a theory 
which we may designate as predetermination. We 
know from our microscopical studies that the germ 
cells possess a certain amount of organization, and 
that the zygote contains certain structures con- 
tributed by the egg and other structures brought into 
the egg by the spermatozoén. Hence, to a certain 
extent, development is predetermined, since the initial 
structure of the zygote determines the characteristics 
of the individual that arises from it. On the other 
hand, development is also epigenetic, and our modern 
conception includes certain features of each theory. 
Tue Creu. A brief account of the structure, 
physics, and chemistry of the cell will serve to give us 
some idea of the condition of the zygote from which 
the individual arises, and will help us to understand 
certain events in the germ-cell cycle to be discussed 
later. 
The cell is the simplest particle of matter that is 
able to maintain itself and reproduce others of its 
kind. The term ‘cell’ was applied by Hooke in 1665 
to the cell-like compartments in cork. Cells filled 
