Facts and Factors of Development 17 



together to form the two daughter nuclei. The cell body then 

 divides by a process of constriction into two daughter cells (Fig. 



7, 1, /) 



The formation, splitting and separation of the chromosomes 

 is the most constant and characteristic feature of indirect nuclear 

 division, but there are other important features which must now 

 be mentioned. In all animals and in many of the lower plants 

 there is present in the cell-body just outside the nuclear mem- 

 brane a small deeply-staining granule, the centrosome, which is 

 usually surrounded by radiating lines. In the early stages of 

 mitosis this granule divides into two which move apart until they 

 come to lie on opposite sides of the nucleus (Fig. 6, A-C). When 

 the nuclear membrane dissolves the radiating lines which sur- 

 round these two centrosomes increase greatly in length forming 

 two asters and those rays which run- through the nuclear area 

 constitute a spindle with the chromosomes in its equator and the 

 centrosomes at its two poles (Fig. 6, D-F). Later the chromo- 

 somes move along the spindle toward its poles where the daugh- 

 ter nuclei are formed. The centrosomes, asters and spindle, 

 known collectively as the amphiaster, constitute an apparatus for 

 the accurate separation of the daughter chromosomes and for the 

 division of the cell-body. 



The chromosomes are most compact and deeply-staining at 

 the metaphase or equatorial plate stage of division; after they 

 have moved to the poles of the spindle they begin to absorb 

 achromatin from the surrounding plasma thus swelling up and 

 becoming chromosomal vesicles with clear contents and chromatic 

 walls (Fig. 8, E, F). These vesicles then continue to enlarge and 

 their chromatin takes the form of threads or granules. After 

 the formation of the daughter nuclei the different vesicles are so 

 closely pressed together that it is usually impossible to see the 

 partition walls between them; however in several different ani- 

 mals and plants the chromosomal vesicles are recognizable even 

 in the resting nucleus (Fig. 8, G), and in every organism the same 

 number of chromosomes, having the same relative shapes and 



