Behavior of the Germ-cells 183 
sometimes sterile. This sterility is due apparently, in some cases 
at least, to irregularities in the division of the germ-cells. When 
we recall that at one stage in the development of the germ-cells 
there may be a pairing and subsequent fusion of the ma- 
ternal and paternal chromosomes, we can readily imagine that 
any differences in their behavior at this time might lead to 
disastrous results. 
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