Yeah see the last two boxes one species has 28 chromosomes and other and 14 chromosomes, each of them gives one chromosome from every pair means the one with 28 gives 14 and the one with 14 gives 7 so both chromosomes combine and make 21 that is the haploid number so the diploid number is 42 (21*2) which is sterile as shown in the box.. <-- This was if the doubling of chromosomes didnt occur in the beginning had it occured then the chromsome number would have doubled as from the two boxes (28*2= 56 and 14*2 =28) and then the halving would have occured and this is what normally happens.but i don't get it, why if the chromosome number didn't double during interphase , the hybrid would be sterile?