THE FROG : HISTOLOGY, GERM CELLS, DEATH 105 



clear membrane is breaking up and the nucleus is under- 

 going certain internal changes. The linin with its 

 contained chromatin granules arranges itself into a coiled 

 thread, the skein or spireme, which then proceeds to break 

 up into a number of equal portions known as chromo- 

 somes, of which the same number appears in every 

 nuclear division in the body (but not, as we shall see, 

 in the germ cells). In the frog this number is (twenty-four. 



Fig. 65. — Karyokinesis. — After Flemming. 



1. Coil stage of nucleus : c.c. , centrosome. 



2. Division of skein into U-shaped loops or chromosomes, and longi- 



tudinal splitting of these. 



3. 4. Recession of chromosomes from the equator of the cell. 



5. Nuclear spindle, with chromosomes at each pole, and achromatin 



threads between. 



6. Division of the cell completed. 



At this stage there appears, stretching from one centrosome 

 to the other across the site of the nucleus, a structure 

 known as the spindle, which consists of fine threads 

 diverging from each centrosome to the equator of a 

 spindle-shaped figure. The chromosomes now become 

 attached to the spindle fibres in a ring round the equator. 

 The next stage consists in the division of each chromo- 

 some lengthwise into two equal halves and the passage 

 of the halves or daughter chromosomes along the threads 



