VI. THE PROCESS OF CELL DIVISION 



All cells arise from pre-existing cells by a process of division. In a few cases 

 this is a simple splitting, or direct division, but in the vast majority of cases the 

 cell divides by a complicated process, known as indirect division, mitosis, or 

 karyo kinesis. In mitosis, both the nucleus and the cytoplasm are involved in a 

 complex and remarkable behavior. 



The following materials have been found to be the most favorable for the 

 study of mitosis: the developing eggs of Ascaris, a parasitic worm; the root tips 

 of plants, chiefly the onion, the hyacinth, or Tradescantia; the developing eggs of 

 fish. A complete outline is provided below for the study of mitosis in the eggs 

 of Ascaris; and following this are brief statements regarding the other materials 

 and the points in which they differ from Ascaris. 



A. MITOSIS IN THE EGGS OF Ascaris 



Ascaris megalocephala, commonly used for this purpose, is a parasitic round 

 worm found in the intestine of the horse. (A description of this animal is given 

 in Hegner, p. 160.) The fertilized eggs pass down the long oviducts of the 

 worm, dividing as they go. Obviously by cutting longitudinal slices through 

 the oviducts at the proper levels, eggs in all stages of division will be obtained. 

 The slides bear such longitudinal slices of the oviduct. 



Examine the slide " A scaris mitosis" with the low power. Identify in each 

 long slice upon the slide the thin walls of the oviduct, composed of large epithelial 

 cells, and its wide cavity completely filled with round objects, each of which is 

 an egg inclosed in a thick shell. Examine one of the round objects with the 

 high power and get a clear idea of what you are looking at. Identify in each 

 one the thick shell inclosing a cavity in which floats the egg cell, considerably 

 smaller than the cavity. The egg has been fertilized and hence possesses two 

 nuclei, its own nucleus and the nucleus from the sperm. Its cytoplasm is 

 vacuolated, that is, appears to contain a number of empty spaces. Examine 

 the egg cells with your highest power and look for each of the following stages 

 of mitosis (see Hegner, p. 29). Considerable searching may be required to find 

 the various stages. Have the assistant help you. As the sections are very thin, 

 only one nucleus may appear, or only parts of the mitotic apparatus may be 

 present. Avoid drawing such partial pictures. Make your drawings large 

 and detailed. 



i. The resting cell. In the so-called resting state, that is, the condition 

 before mitosis begins, the egg presents the same appearance as other cells which 

 we have studied. It contains two 1 nuclei, its own and the sperm nucleus. Each 



'This is the case only in the fertilized egg cell, not in other cells. 



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