REPRODUCTION AND DEVELOPMENT 247 



completely separated, the daughter organisms becoming a part of 

 the colony. 



Similar conditions are found among the single cell plants. In the 

 yeast plant a modified form of cell division occurs, in which the 

 new organism is distinctly smaller than the parent, a characteristic 

 known as budding. The buds may remain attached to the parent 

 cell and themselves give rise to buds. Thus a colony of yeast cells 

 may be formed by the budding of daughter cells that adhere to the 

 parents (Fig. 24). When, as in a paramcecium, the two daughter 

 cells are equal in size and both have the characters of young animals, 

 the parent disappears in the reproductive process. If one of the mem- 

 bers is distinctly smaller than the other and it alone has the char- 

 acters of a young organism, as in the yeasts, one may distinguish 

 between parent and offspring. 



Agamic reproduction occurs quite commonly in many phyla of the 

 Metazoa. In the diploblastic forms, Porifera and Coelenterata (Figs. 

 32 and 33), buds are formed in the body wall; in colonial types 

 these may continue to develop attached to the parent, thus forming 

 colonial groups. Colonial forms may therefore be thought of as 

 incomplete or partial agamic reproductions. 



Agamic reproduction among the free-living Platyhelminthes is 

 accomplished by the fission or fragmenting of the parent body. This 

 process is illustrated in a most striking fashion by the life histories 

 of two rather common fresh water planaria. In the Spring one may 

 find, in water-filled ditches and pools of the Middle West, consid- 

 erable numbers of a small, active planarian, Planaria velata. When 

 brought to the laboratory and kept in basins of water for some 

 time the animals become less active. An individual creeps slowly 

 across the floor of the container. The tail is broken ofiF and left be- 

 hind; then another fragment breaks of? from the posterior end; 

 then another, until only the head of the original animal remains. 

 Eventually the head dies. Each fragment secretes a capsule as a 

 covering and within this capsule the tissues of the fragment develop 



