MITOSIS IN ANIMAL CELLS 79 



faintly visible. E shows the commencement of the cell-plate 

 (c.p.) across the middle of the spindle, and F the two young 

 daughter cells each with a new nucleus in which the chromo- 

 somes have again broken up into granules. 



It is only by the examination of large numbers of examples 

 that all the minute details of the process can be elucidated, but 

 the main features as represented in the above figures can very 

 easily be made out. 



For comparison with the process of mitosis as seen in typical 

 plants such as Galtonia, we may take the first division of the 

 fertilized egg in the horse-worm, Ascaris, a classical subject from 

 the study of wbich much of our knowledge of nuclear division in 

 animal cells has been derived. In this case there are only four 

 chromosomes, but they are large and conspicuous, and charac- 

 teristically V-shaped when forming the equatorial plate on the 

 spindle. 



Fig. 35 is again taken from actual photographs. In this 

 figure, A represents a side view of the entire egg-cell during 

 the division of the nucleus, with spindle (sp.), asters (as.), 

 centrosomes (one only of which, cs., appears in the photograph, 

 the other being out of focus), and equatorial plate (acq.). 13 

 shows a similar stage viewed from one pole, so that the spindle 

 itself does not appear, but the four chromosomes forming the 

 equatorial plate are distinctly visible. C shows the two daughter 

 cells or blastomeres resulting from the first division of the egg, 

 each with the nucleus preparing for further division, and I) 

 represents a later stage in which the nucleus of each daughter 

 cell is again actually in process of division and shows the 

 separate chromosomes very distinctly. 



It must not be supposed that the phenomena of mitosis are by 

 any means confined to the higher animals and plants ; they are 

 observable throughout the animal and vegetable kingdoms, in 

 unicellular as well as multicellular forms. The process has long 

 been known to take place, for example, in at any rate some 

 Amcebre, and it probably occurs wherever there is a clearly 

 differentiated nucleus. The Bacteria and their allies, in which 

 the chromatin granules are scattered throughout the cell body 

 and there is no proper differentiation into cytoplasm and nucleus, 

 apparently form exceptions to the general rule. 



There are, however, even amongst the higher animals, some cases 

 of cell-division which do not exhibit mitotic phenomena, but in 



