CHROMOSOME GROUPS IN SYROMASTES. 2O3 



chromosome of Anasa, Protenor and other similar forms, and 

 that the small idiochromosome has disappeared. Payne discov- 

 ered that in the reduvioids, where a single small idiochromo- 

 some or " Y-element " 2 is always present, the large idiochromo- 

 some is in some species a single chromosome (Diplocodus}, in 

 others is represented by two (Fitchia, Conorhinus} or three chro- 

 mosomes (Prio nidus') , and in the galgulid genus Gelastocoris 

 (Galgulus} by four chromosomes, which behave in maturation as 

 a single unit (X-element) that is obviously comparable to a single 

 large idiochromosome in its relation to sex-production. Payne 

 concludes, with great probability, that the double or multiple 

 X-element in these forms has arisen by the separation of an orig- 

 inally single large idiochromosome (such as still exists in related 

 species) into two or more components. In Conorhinus, where 

 the X-element is double, the two components are unequal in size, 

 and by the disappearance of the Y-element a condition would 

 arise closely similar to that seen in Syromastes. 



Whether such has been the actual mode or origin in Syro- 

 mastes or not, it seems probable that here too the double " acces- 

 sory " was originally a single chromosome that has separated into 

 two parts, which still act as a unit in the maturation divisions and 

 retain the same relation to sex-production as the original one. 

 Phylloxera carycccaulis may plausibly be regarded as in process 

 of transition from the condition in which a single " accessory " 

 chromosome is present (as appears to be the case in the aphids) 

 to one in which it has separated into two parts, as in Syromastes. 



1 will add a brief account of the female groups in Pyrrochoris 

 aptcrus L., material for which has also been obtained through 

 Professor Boveri. A reexamination of the male groups (Wilson, 

 Study IV.) showed the spermatogonial number to be 23, including 

 a single unpaired idiochromosome ("accessory" chromosome) 

 which is at once recognizable from the fact that it is nearly twice 

 the size of any of the other chromosomes. This passes into half 

 the spermatozoa, which receive 12 chromosomes, while the others 

 receive but n, as was originally described by Henking. The 



2 In a recent general discussion I have used the terms " X-element " and 

 "Y-element" to designate respectively the large and small idiochromosomes, 

 or their homologues, whether they consist of a single, chromosome or of more 

 than one. See Science, XXIX., 732, January 8, 1909. 



