PROrOPLASM AND THE CELL. 11 



The spindle-fibres of the centrosome then appear to penetrate 

 through the nuclear membrane, which sooner or later disap- 

 pears, and by their growth push the chrpmatin skein towards 

 the equator of the nucleus, the skein at the same time break- 

 ing into a number of fragments, termed chromosomes . The 

 number of these chromosomes is practically constant for the 

 cells of any species of animal, and though there is consider- 

 able variation in different species, yet in the majority of ob- 

 served cases the number belongs either to the series 2, 4, 8, 

 16, 32, or to that of 6, 12, 24. They vary considerably in size 

 in different forms, being in some cases V-shaped or in others 

 dumbbell-shaped, and arrange themselves finally in a more or 

 less definite ring surrounding the equator of the nucleus. At 

 this stage, which is known as the equatorial-plate stage, the 

 appearance presented in Figure 2, B, is found. At each pole 

 of the nucleus is a centrosome surrounded by the astral rays 

 and with the spindle-fibres extending towards and coming 

 in contact with the chromosomes lying at the equator of the 

 nucleus, and to *the entire complex the term amphiaster is 

 sometimes applied. 



In the next stage the V-shaped chromosomes, to take this 

 as a typical shape, which are arranged with the apex of the 

 V towards the nuclear axis, divide longitudinally. Assuming 

 that there were originally six chromosomes in the equatorial 

 plate, as the result of the division there are now twelve ar- 

 ranged in pairs (Kg. 2,,C'). One of each pair, now proceeds 

 to moveHowards one of' the poles, of the nucleus and the other 

 to the other, so that eventually near each pole there is a 

 group of six chromosomes, and between the two groups there 

 may be seen stretched a number of connecting fibres identical 

 in appearancS^with the original spindle-fibres, while in some 

 cases at the equator of the egg there is to be seen on these 

 fibres a number of darkly staining dots which may be termed 

 the intermediate bodies (Fig. 2, D). At about this stage the 

 cytoplasm bf'gins to divide, the plane of its division passing 

 through the equator of the nucleus, and there are thus formed 

 two cells, each containing a nucleus composed of six chromo- 

 somes and a centrosome. The chromosomes now begin to 

 become irregular in shape, they gradually fuse and are finally 



