MITOSIS 



As the cellular nature of the fundamental tissues found in the 

 bodies of higher animals becomes apparent there naturally arises 

 the question of how these tissues increase in amount or bulk to 

 account for the increase in the size of the animal during growth. 

 Since the cells of any given tissue do not increase in size beyond a 

 certain maximum it seems reasonable to accept the explanation 

 that increase in the size of an animal is due to an increase in 

 the number of component cells. The manner of cell increase 

 is that of cell division. A given cell at the proper time divides 

 into two daughter cells which grow to the normal size and in 

 their turn divide. In most tissues the method is that known as 

 indirect cell division or mitosis. 



Growing tissues of almost any organism might be used to illus- 

 trate indirect cell division, but for several reasons the developing 

 eggs of Ascaris megalocephala, a parasitic roundworm, are 

 recommended. 



In the female Ascaris the reproductive organs are so modified 

 as to comprise various specialized parts one of which is the uterus 

 or part retaining the eggs during development. Sections through 

 the appropriate portion of the uterus show the eggs (cells) in 

 various stages of mitosis. Such sections should be mounted 

 in rows lengthwise of the microscopic slide. A well-established 

 method is that of arranging sections showing maturation, ferti- 

 lization, and mitosis in five rows across the sHde, the fifth or 

 bottom row containing cells in a state of active mitosis. What- 

 ever the arrangement it should be explained by the instructor 

 by way of saving time for actual study. 



If Ascaris material is used, examine the section of the fifth 

 row on the slide with the low power of the compound miscroscope 

 and identify the thin walls of the uterus composed of large epithe- 

 lial cells and its wide cavity completely filled with circular 

 objects, each of which is a section of an egg or true cell. Examine 

 one of the eggs under high power and get a clear idea of its struc- 

 ture. Identify the thick shell or capsule inclosing a cavity 

 occupied by the egg cell which is considerably smaller than the 



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