CHROMOSOME-NUMBER IN NOTONECTA. 23 



stages do not exist. Payne 1 has concluded that the multiple 

 X-element in the Reduviidae has arisen from a single X-element 

 of an ordinary idiochromosome-pair by a separation into two or 

 more parts. Wilson 2 has accounted for the double accessory in 

 Syromastes by assuming that it has arisen from a single chromo- 

 some that has split into two parts. These hypotheses are sus- 

 tained by the evidence of Notonecta, two species of which show 

 permanent fusion and separation, and the third species the inter- 

 mediate condition. 



Besides the interesting correlation of the change in chromo- 

 some-number and change in species in Notonecta, the condition 

 of temporary fusion and separation of two chromosomes in one 

 species, TV. insulata, is of considerable interest, especially in com- 

 parison with the facts found in other insects. Sinety 3 has found 

 in the Phasmidae and McClung 4 in other orthoptera that the 

 accessory is sometimes attached to another chromosome ; in Mer- 

 meria, McClung found this compound chromosome further as- 

 sociated with another chromosome. Wilson 5 describes in Meta- 

 podius a coupling of the supernumeraries with the idiochromo- 

 somes during part of the maturation-process. Wallace 6 has found 

 in the spider that the accessory is made up of two components, 

 sometimes united, sometimes separate. Similarly, in Syromastes. 

 Wilson 7 has found that the accessory consists of two unequal 

 elements which are closely united in the maturation-divisions. 

 In the Phylloxerans, Morgan 8 describes two accessories, some- 

 times separate, sometimes united. It is to be noted, however, 



! Payne, F., 1909, "Some New Types of Chromosome Distribution and Their 

 Relation to Sex," BIOL. BULL., XVI., 3, 4. 



2 Wilson, E. B., 1909, "The Female Chromosome Groups of Syromastes and 

 Pyrrochoris," BIOL. BULL., XVI., 4. 



3 Sinety, R. de, 1901, "Recherches sur la Biologic et 1' Anatomic des Phasmes," 

 La Cellule, XIX. 



4 McClung, C. E., 1905, "The Chromosome Complex of Orthopteran Sperma- 

 tocytes," BIOL. BULL., IX., 5. 



6 Wilson, E. B., 1909, "Studies on Chromosomes," V., Journ. Rxp. Zoo/., VI., 2. 



6 Wallace, L. B., 1909, "The Spermatogenesis of Agalena ncevia," BIOL. BULL., 

 XVII. , 2. 



7 Wilson, E. B., 1909, "Studies on Chromosomes," IV., Journ. Rxp. Zool., 

 VI., i. 



8 Morgan, T. H., 1909, "A Biological and Cytological Study of Sex Determination 

 in Phylloxerans and Aphids," Journ. Rxp. Zool., VII., 2. 



