70 THE CELL. 



According to the opinion of C. Rabl (85), there remains in the 

 nucleus, even after it has fully returned to a state of rest, a polar 

 arrangement of the chromatin loops that is, an arrangement of the 

 axis of the loops in the direction of the centrosphere. The area 

 toward which the crowns of the loops point is known as the polar 

 field. 



The equatorial differentiation of the connecting threads of the 

 central spindle, above mentioned, was first observed in vegetable 

 tissue, and is known as the cell-plate. (Fig. 29.) In animal cells such 

 a plate is relatively rare, and, when seen, is found developed in a 

 rudimentary form (v. Kostanecki 92, I). 



(d] Telophases (M. Heidenhain 94). In these phases of mitosis 

 the cell divides completely. The daughter nuclei and centrospheres, 

 which do not yet occupy their normal position in the daughter cells, 

 show movements that result in their assuming their normal positions. 



From our description it is seen that the anaphases represent the 

 same stages as the prophases, only in an inverted sequence. In the 

 latter case, the result is the resting nucleus, while the prophases lead 

 to the metaphases. 



The fertilized ovum also divides by indirect nuclear division. 

 (Figs. 22-29.) From it are derived, by this process, the seg- 

 mentation cells, or blastomeres, from which the whole embryo is 

 developed. 



(e) The Heterotypic Form of Mitosis. The above-described 

 type of indirect or mitotic nuclear division (Jwmcotypic mitosis] is 

 the usual one. Variations, however, occur, as, for instance, in the 

 so-called Jieterotypic form of division (Flemming 87), which occurs 

 in certain cells of the testes (spermatocytes). In this form the first 

 stages are lacking, the nucleus possessing from the beginning a 

 skein-like structure. The longitudinal splitting and division of the 

 chromatin threads take place during the first spirem stage, after which 

 there is a phase in which the figure may be compared with an aster 

 of ordinary mitosis, although the free ends of the threads in this 

 case are seldom observed. The latter is due to the fact that after 

 the longitudinal splitting, the ends of the chromosomes remain 

 united, or, if entire separation occurs, they are again joined. In this 

 way closed loops are formed extending from pole to pole. Later 

 the threads break at the equator and move toward the poles, again 

 dividing to form the daughter stars. 



2. AMITOSIS. 



Very different from the indirect form of nuclear division is the 

 direct or amitotic. It appears to occur seldom as a normal process, 

 and is only exceptionally followed by a subsequent cell-division 

 (ind. Flemming, 91, III). As a consequence, this process, in most 

 cases, results in the formation of polynuclear cells (polynu clear leu- 

 cocytes, giant-cells, etc.). The complicated nuclear figures of 



