8 TEXT-BOOK OF EMBRYOLOGY. 



ings along the plane of future division of the cell, forming what is known as the 

 mid-body or cell-plate. This splits into two layers, between which the division 

 of the cell takes place. The formation of a distinct cell-plate in animal 

 mitosis is rare. In place of this there is a modification of the cytoplasm along 

 the line of future division, sometimes called the cytoplasmic plate. 



As to what may be called the dynamics of mitosis, there has been much 

 controversy, but comparatively little has been definitely settled. 



It' would appear that in most cases the centrosome is the active agent in 

 initiating, and possibly in further controlling the mitotic process. Boveri, 

 for this reason, refers to the centrosome as the "dynamic center" of the cell. 

 The centriole first divides into two, around each of which an astral system of 

 fibers is formed. The origin of these fibers appears to differ in different cells. 

 Thus, in some cases — Infusoria, for example — the centrosome lies within the 

 nucleus and the entire mitotic figure apparently develops from nuclear struc- 

 tures. In some of the higher plants both central spindle fibers and asters 

 are formed from the spongioplasm. In still other cases — for example, the eggs 

 of Echinoderms — part of the figure (the asters) is developed from the cytoplasm, 

 while the fibers of the central spindle are of nuclear origin. 



It must, however, be admitted that centrosome activity is not absolutely 

 essential to cell division, for there are cases in which division of the chromo- 

 somes occurs without division of the centrosome, while in the higher plants 

 mitosis occurs, although no centrosome can be distinguished at any stage of 

 the process. 



The behavior of the centrosome before, during and after mitosis varies in 

 different cells. In some cells the centriole is apparently an integral part of 

 the cell, persisting throughout the resting stage. With it may remain more 

 or less of the aster, the whole constituting the already mentioned attraction 

 sphere. In other cells — for example, mature egg cells — the centriole with 

 its fibrils apparently entirely disappears during the resting stage. 



In regard to the origin of the chromatic portion of the mitotic figure, no 

 difference of opinion exists, so evidently does it arise, as already noted, from the 

 chromatic portion of the nuclear reticulum. Its destination in the nuclear 

 reticulum of the daughter cells is equally well established. The details of the 

 formation of the chromosomes vary. Thus in some cases there is no single 

 spireme thread, the spireme being segmented from its formation, each segment 

 of course corresponding with a future chromosome. In other cases no spireme 

 whatever is formed, the chromosomes taking origin directly from the nuclear 

 reticulum. In still other cases the spireme while yet a single thread splits 

 longitudinally so that there are two threads present, the transverse divisions 

 into chromosomes taking place subsequently. 



As to the time required for the mitotic process, considerable variation exists 



