6o 



CELL-DIVISION 



2. Heterotypical Mitosis 



Under this name Flemming ('87) first described a peculiar modi- 

 fication of the division of the chromosomes that has since been shown 

 to be of very great importance in the early history of the germ-cells, 



D 



Fig. 26. Heterotypical mitosis in spermatocytes of the salamander. [FLEMMING.] 

 A. Prophase, chromosomes in the form of scattered rings, each of which represents two 

 daughter-chromosomes joined end to end. D. The rings ranged about the equator of the spindle 

 and dividing; the swellings indicate the ends of the chromosomes. C. The same viewed from the 

 spindle-pole. D. Diagram (Hermann) showing the central spindle, asters and centrosomes, and 

 the contractile mantle-fibres attached to the rings (one of the latter dividing). 



though it is not confined to them. In this form the chromosomes 

 split at an early period, but the halves remain united by their ends. 

 Each double chromosome then opens out to form a closed ring 

 (Fig. 26), which by its mode of origin is shown to represent two 

 daughter-chromosomes, each forming half of the ring, united by 



