160 



PHYSICAL BASIS OF HEEEDITY 



the chromosome is ^ ^ shortened" by an amount correspond- 

 ing to the *^ length'' of the deficient region. 



It is not without interest to notice that in the first 

 case the duplicating piece is attached to that end of the 

 first chromosome where the spindle fibre is attached. In 

 the other case the duplicating piece is attached to the 



a b 



Fig. 64. — An egg of Ascarishivalem fertilized by sperm of A. univalens, a; later stage 



of same, b. 



middle of the third chromosome, and in this chromosome 

 the spindle fibre is attached to the middle. 



An interesting case of triploidy has been reported in 

 the threadworm Ascaris (Boveri). Two varieties occur, 

 one with four chromosomes (haploid two), and one with 

 two (haploid one). Rarely a female of one variety is 



•*» 



Fig. 65. — Diploid and haploid groups of the sundew Drosera. (After Rosenberg.) 



found that has mated with a male of the other variety. 

 The fertilized eggs have each three chromosomes (Fig. 

 64). As yet no triploid adults have been met with, so that 

 the method of conjugation of the chromosomes in the 

 triploid types is not known. 



Rosenberg crossed two species of sundew, Drosera 

 longifolia, with 40 chromosomes (haploid 20), and D. 

 rotundifolia, with 20 chromosomes (haploid 10), Fig. 65. 



