78 MANUAL OF PHYSIOLOGY. 



growing from them. If the budlike protrusion be large, nearly 

 equal in size to the cell itself, the process receives the name of 

 fission, or division. In well marked typical fission the parent 

 cell divides into two parts of equal size, each of which becomes a 

 perfect individual. Various gradations may be traced between 

 the two processes, so that it is difficult to draw any very distinct 

 line between budding and fission. The budding and fission may 

 be multiple; many buds and several units, products of division 

 may remain together, and form what is called a colony. When 



Cells of a fungus (Glceocapsa) showing different stages (1-4) of endogenous 

 division. (After Sachs.) 



this multiple budding or division takes place, so that the new units 

 are included within the body of the parent cell, then the pro- 

 cess is called endogenous reproduction or spore formation. As in 

 the gradations between budding and fission, so it is difficult to 

 draw a hard and fast line between what may be called multiple 

 fission and spore formation. 



In tracing the stages of development of the highly differentiated 

 cells of some tissues, we have to pass through a series of changes 

 which form a cycle that may well be called the lifetime of the 

 cell. The duration of this cycle varies greatly in different indi- 

 vidual cells. Some cells are very short-lived, being destroyed in 

 the act of secretion ; others probably endure for the entire life of 

 the animal. The life-history of every cell begins with the stage 

 when it is composed entirely of indifferent protoplasm, in which 

 various modifications are subsequently produced. Let us take as 

 an example a cell of the outer skin or cuticle, and examine its life- 

 history. The cuticle is made up of numerous layers of cells laid 

 one on the other, and the surface cells are constantly being rubbed 

 or worn off. We find that these cells have their origin from the 

 cells of the deepest layer, which is next to the supply of uutri- 



