DETAILS OF MITOSIS 



87 



of the spindle as usual. As will be shown in Chapter V.,the divisions 

 by which the germ-cells are matured are in many cases of this type ; 

 but the primary rings here in many cases represent not two but four 

 chromosomes, into which they afterward break up. 



Pig. 37. — 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. B. The rings ranged about the equator of the spindle 

 and dividing; the svveHings 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). 



2. Bivalent and Pliirwalent CJiromosomes 



The last paragraph leads to the consideration of certain varia- 

 tions in the number of the chromosomes. Boveri discovered that the 

 species Ascaiis nicgalocepJiala comprises two varieties which differ in 

 no visible respect save in the number of chromosomes, the germ-nuclei 

 of one form (" variety bivalens " of Hertwig) having two chromosomes, 



