Studies oil Chromosomes yj 



plate as to undergo an equation division, like the idiochromosomes 

 of other Hemiptera heteroptera. In uniting to form a bivalent 

 before the first division these chromosomes differ from those of 

 most other Hemiptera, but in all other respects up to the end of 

 the first division they correspond exactly with them. But even 

 this difference is bridged by a condition occasionally seen in other 

 forms, for instance in Lygaeus and Metapodius.*^ In the last 

 named form the typical and usual condition is that the idiochro- 

 mosomes are in the first division quite separate, lying eccentrically 

 outside the principal ring of chromosomes like the unpaired idio- 

 chromosomes of other coreids (Photos 6 and 7), and in this posi- 

 tion they separately divide. Exceptionally, however, they lie in 

 close contact (Photos 8 and 9), forming an asymmetrical bivalent 

 precisely like that of Syromastes. In both cases this bivalent 

 divides equationally, giving two asymmetrical daughter-dyads, 

 thus -.. 



J: 



The exactness of the correspondence up to this point seems to 

 leave no doubt of the homology of this pair of chromosomes in the 

 two forms. In the second division, however, the two species show 

 a remarkable contrast. In Metapodius, as in Lygaeus or Euschis- 

 tus, the two idiochromosomes are always united to form an unsym- 

 metrical bivalent which enters the equatorial plate and is separated 

 into its two components, half the spermatids receiving the large 

 one and half the small. In Syromastes, on the other hand, the 

 idiochromosomes remain united and do not enter the equatorial 

 plate at all, but pass directly to one pole where they are included 

 in the daughter-nucleus, as Gross has described (Photos 11 to 

 17). Owing to this behavior of the idiochromosome bivalent, 

 polar views of the second division always show but ten chromo- 

 somes instead of eleven (Photo 10). In this case therefore half 

 the spermatid nuclei receive two more chromosomes than the 

 others, the two classes having respectively ten and twelve chromo- 

 somes. As the idiochromosome bivalent passes to the pole its 

 two components are usually closely united, and often cannot be 



* The latter remarkable genus, which presents the phenomenon of the "supernumerary chromosomes'' 

 (Wilson '07c), will form the subject of a forthcoming fifth "Study." 



