64 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



and a nuclear membrane is formed. Two cells each exactly like the original 

 single cell have thus been produced. 



B. MITOSIS IN PLANT ROOT TIPS 



The root tips of plants grow very rapidly in some cases and hence are favorable 

 places for finding all the stages of mitosis. For this purpose longitudinal sections 

 are made through the root tip. The tip is covered by a root cap consisting of 

 older and hardened cells, which is pushed ahead of the real growing region. The 

 dividing cells are therefore found a little distance back from the tip. 



The process of mitosis in these plant cells differs from that seen in the eggs 

 of Ascaris chiefly in that centrosomes and asters are entirely lacking, so that the 

 mitotic figure consists of spindle only. Further differences are: in the resting 

 stages one or more conspicuous nucleoli will be found within the nucleus; the 

 spireme thread is closely coiled so that at this stage the section of the nucleus 

 is packed with small wormlike segments of the spireme; the chromosomes are 

 numerous and less distinctly U-shaped; in the anaphase the resemblance of 

 each group to the tops of palm trees is quite striking; and in the telophase, the 

 new cell wall is not produced by a constriction, but forms in place, apparently 

 in part from a condensation of the spindle fibers. The spindle is rather faint 

 throughout and is seen best only in the late stages of mitosis. 



C. MITOSIS IN THE EGGS OF THE WHITEFISH 



The sections are taken through the dividing eggs of the fish, generally in 

 the early stages when the cells are quite large. Each section shows a number of 

 cells, some of which will be found to be in various stages of mitosis. Differences 

 from Ascaris are that the chromosomes are quite small, very numerous, and 

 rodlike in form; and that a number of minute centrosomes instead of the one 

 large centrosome of Ascaris are present. They are difficult to see. A clear 

 field surrounds the place occupied by the centrosomes, and from this the long 

 fibers of the asters radiate nearly filling the cell. Fish eggs are valuable for the 

 study of mitosis chiefly because the mitotic figure is of such large size and so 

 distinctly fibrillar in them that they present a striking appearance, which closely 

 corresDonds to the textbook representations of this structure. 



