SPECIAL PROBLEMS OF MEIOSIS 77 



V's and three pairs of rods). There is no pairing of 

 chromosomes at zygotene and all ten behave as 

 univalents. The spindle which forms at the prometa- 

 phase of the first meiotic division is a half- spindle 

 (see Fig. 3) to which both the large V-shaped chromo- 

 somes and one member of each of the other pairs 

 are attached. The remaining chromosomes move 

 away from the half-spindle ; there is genetic evidence 

 that these four chromosomes which are unattached 

 to the half -spindle are all paternal chromosomes. 

 The first meiotic division thus separates a group of 

 six chromosomes from a group of four — the latter 

 degenerate in a small bud of cytoplasm which becomes 

 cut off from the main cell like a polar body, while 

 the group of six chromosomes proceed to the second 

 meiotic division. Here a normal bipolar spindle is 

 formed and five of the six chromosomes divide 

 normally, but one of the rod-shaped chromosomes 

 divides in such a way that both its halves go to the 

 same pole. The group of chromosomes at this pole 

 form a sperm nucleus, while the other group degener- 

 ates. Only one sperm is thus formed from each 

 primary spermatocyte and it contains more than the 

 haploid number of chromosomes. ^^5 ^ complicated 

 mechanism involving the elimination of certain 

 chromosomes occurs during the cleavage divisions.*' 

 It is clear that this type of meiosis is even more 

 highly modified from the normal type than that 

 found in Drosophila, and that it involves a complete 

 suppression of crossing-over in the male. In the 

 female Sciara, on the other hand, meiosis is entirely 

 normal. A similar type of meiosis appears to occur 

 in the male Hessian Fly {Phytophaga destructor). ^^^ 



Genetically Determined Abnormalities of 



Meiosis 



A fairly large number of gene -mutations are now 

 known which lead to abnormaHties of meiosis. Most 

 of these mutations interfere with chiasma -formation, 



