VARIATION OF CHROMOSOMES 163 



double set AA with B not producing any striking differ- 

 ence from the single set A^B. When this second hybrid 

 {3N) matures its germ-cells, the two homologous series 

 {A -\- A) mate with each other, and then segregate at the 

 first division, while the unmated J?-series simply divides. 

 At the second division both the A- and the Z?-series divide, 

 thus giving to each sperm a haploid set of chromosomes 

 {A -{- B). The sperm then is the same as the sperm of 

 the first hybrid. So long as the back-crossing continues 

 the outcome is expected to be the same. 



If, instead of back-crossing the first hybrid to parent 

 A, it is back-crossed to parent B, the same result as 

 before takes place, except that the second hybrid is now 

 A,-\- B + B. When it matures its germ-cells, the B's 

 unite and then separate, giving AB sperm as before. 



Here then we find a kind of inheritance that super- 

 ficially appears to contradict the generality of Mendel's 

 law of segregation. On the contrary, a knowledge of the 

 chromosomal behavior shows that the results are different 

 because the mechanism of conjugation of the chromosomes 

 is changed, and changed moreover in such a way that on 

 the chromosome theory itself the results are what are to 

 be expected. 



These crosses are so important that some further 

 details maly be added. The whole (2N) and half (IN) 

 number of chromosomes of the three species studied by 

 Federley are as follows : 



Whole Half 



Pygaera anachoreta 60 30 



Pygaera curtula 58 29 



Pygaera pigra 46 23 



In the hybrid between the first two species the number of 

 spermatocyte chromosomes was found to be 59 (30 + 29). 

 No union between any of the maternal and paternal chro- 

 mosomes could have taken place. But in the hybrid 

 formed by the union of the two more nearly related spe- 

 cies, curtula and pigra, the number of spermatocyte chro- 



