Studies on Chromosomes. 399 



4 



gonial chromosomes is an even one (as in Anasa and Syromastes); 

 and there seems to be no escape from the conclusion that the acces- 

 sory IS here a bivalent body arising by the synapsis of two equal 

 spermatogonia! chromosomes. Even in cases showing an odd 

 number of spermatogonia! chromosomes (as in many Orthoptera 

 and some Hemiptera — for example Alydus or Protenor) it has 

 been assumed, and with good reason, that one of the chromosomes 

 (probably the accessory) is already bivalent,^ and Montgomery 

 has shown ('01, i) that in Protenor the large accessory ("chromo- 

 some X ") is sometimes transversely constricted into two equal 

 halves in the spermatogonia. A similar fact was subsequently 

 shown in l^armostes ('01, 2) which also has normally an odd sper- 

 matogonia! number. To this should be added the fact that these 

 forms possess the small bivalent central chromosome (which 

 arises by the synapsis of two equal microchromosomes) in addition 

 to the accessory. The difficulty pointed out above cannot be 

 escaped by supposing that the disappearance of one of the idio- 

 chromosomes has been effected by its gradual absorption by the 

 other; for this assumption, too, fails to explain the even number 

 of spermatogonia! chromosomes. Apparently therefore the hypo- 

 thesis I have suggested* must in the present state of our knowl- 

 edge be considered untenable.^ 



It appears more probable that the idiochromosomes are com- 

 parable to the two equal microchromosomes or "chromatin- 



^> 



'C/. Montgomery, '04. 



-Since this paper was sent to press I have determined beyond the possibility of doubt, I think, that 

 the number of spermatogonia! chromosomes in Anasa tristis is 21, not 22 as given by both Paul- 

 mier and Montgomery. This result is based on the study of a large number of preparations, and 

 careful camera drawings of more than twenty perfectly clear metaphase figures have been made. All 

 without exception show 21 chromosomes, and I have sought in vain for even a single cell that shows 

 22. (Paulmier's original slides were used.) If corroboratory evidence be needed it is given by the 

 fact that there are always three macrochromosomes, one of which is obviously without a mate of like 

 size, and is probably the accessory. I have, also, positively determined the spermatogonial number 

 to be 21 in a form included in Paulmier's material and labeled " Chariesterus antennator," (since 

 this number disagrees with Montgomery's count of the spermatocytes there may be an error of iden- 

 tification; but the form is certainly different from .'Knasa) and 15 in Archimerus calcarator (from my 

 own material, identified by Mr. Uhler), both members of the same family as Anasa. This wholly 

 unexpected result perhaps justifies a certain skepticism in my mind in regard to the accounts of other 

 observers, who give an even spermatogonial number for forms possessing an accessory chromosome; 

 and if this be well founded the objection urged above disappears. I shall return to this subject 

 hereafter. It is needless to say that had I been acquainted with these facts, the discussion that 

 follows would have been different. 



