6 



DEVELOPMENT OF THE FROG'S EGG 



[Ch, I 



halves of the chromosomes separate from each other along the 

 longitudinal line of division. The process is similar to the ring- 

 formation of Gryllotalpa. In this way twelve loops are formed 

 from the twelve chromosomes. The bent ends of the new loops 

 or rings correspond to the middle portions of the earlier rods or 

 chromosomes (see the + and — signs in Fig. 3, A, C). Mean- 

 while the achromatic spindle between the centrosomes has devel- 

 oped, and the loops of chromatin are arranged on the threads of 

 the spindle, as seen in Fig. 3, B. At the next stage each loop 



B 



mm 



Fig. 3. — Heterotypic type of nuclear division in Salamandra. (After Flemming.) 



breaks at the equator, i.e. at the point where the ends of the 

 rods fused at an earlier period, and begins to migrate toward 

 its centrosome (Fig 3, D). While this migration of the twelve 

 bent chromosomes is taking place, each chromosome may be 

 seen again to split longitudinally, although the two halves 

 remain in contact (Fig. 3, E). The cell then passes into a 

 resting-stage. 



In the homoeotypic division the first phase, the spireme, is simi- 



