PROBLEM I . Hoiv Simple Animals ajid Plants Reproduce 



413 



B 



Fig. 361 Five stages in the reproductioii of an ameba. How does B differ frotn A? 

 What changes are shown in C? What is happening in D? 



ing in its pseudopods; it becomes less 

 active. The nucleus comes to rest near 

 the center of the cell and divides. While 

 the nucleus is dividing, the cytoplasm 

 streams in opposite directions away from 

 the center of the cell. The two new 

 nuclei move away from one another 

 along with the cytoplasm. The two re- 

 sulting parts then seem to pull away 

 from each other. The cytoplasm con- 

 necting them becomes thinner and thin- 

 ner until finally it is only a thread, which 

 breaks. In about half an hour one ameba 

 becomes two amebae, each with its own 

 nucleus. The new amebae rapidly gro"w 

 to full size. 



This division of the nucleus and the 

 separating of the cell body into two 

 equal parts in a one-celled organism is 

 a process known as binary fission (by'na- 

 ree fish'un). In binary fission, as you just 



read, the parent disappears; the whole 

 organism makes the two offspring. These 

 two are often spoken of as the daughter 

 cells and the parent as the mother cell. 

 Strictly speaking, there are neither 

 daughters nor mothers; but no better 

 names have been found. 



Binary fission occurs not only in ame- 

 bae and Paramecia but in other protozoa 

 and in many single-celled plants. Note 

 that cell division in a many-celled organ- 

 ism is not called binary fission because 

 new organisms do not result. The term 

 binary fission is reserved for the repro- 

 duction of a single-celled animal or plant. 



A variation from binary fission. An 

 equally simple but far less common 

 method of reproduction takes place in 

 the yeast plant, many thousands of 

 which make up a cake of yeast. In re- 

 production, the nucleus moves to one 



Bud forming 



Bud forming 



Fig. 362 The comvion yeast used for baking. How many cells are in the process of 

 budding? Where is the nucletis when the cell is budding? 



