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 undiff erentiated 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 spermatozoon. 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. 



THE CELL. 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 



