GENERAL ANATOMY. 



69 



somes, while from these poles fine threads, the spindle-fibres, 

 run to the centre or equator of the 

 nucleus. These fibres are in many 

 cases certainly derived from the 

 achromatic nuclear reticulum, while 

 in others a greater or less part in 

 their formation is taken by the 

 protoplasm. A debated point is the 

 relations of the fibres in the equa- 

 torial plane of the spindle. Do all 

 the fibres extend from pole to pole ? 



Do all Of them end in the equatorial FIG. 21. Spindle formation imd divi- 

 sion of the centrosomes in Ascaris 



plane, SO that the Spindle Consists of megalocephala. (After Brauer.) c, 



centrosomes ; c?i, chromosomes. 



two cones ot fibres separated at the 



equator ? Or, lastly, are fibres of both kinds present in the same 

 spindle ? It would appear that differences exist in these respects 

 in different objects. 



All of the chromatin of the nucleus lies in the equator, united 

 in the ' equatorial plate/ but by this must not be understood a 

 connected mass but a layer of separate bodies, the chromosomes, 

 for the chromatin of the nucleus divides early into particles which 

 are rarely spherical or rodlike, but usually have the shape of 

 U-shaped loops. These chromosomes are of equal size in the same 



FIG. 22. Cell division in the skin of Salamandra maculosa. (After Rabl.) 



cell, and, what is of greater theoretical significance, their number 

 is identical in all the cells of all the tissues of one and the same 

 species. 



