STUDY OP CHROMOSOMES IN NOTONECTA 123 



and X which is next in size. There are six small chromosomes 

 corresponding to the one in the center in the first division, and 

 two small peripheral ones; Y is one of the next smallest. 



In N. insulata there are two types of first division groups, 

 one with 14 chromosomes including two small ones in the cen- 

 ter, like N. indulata, and the other type with 13 chromosomes 

 including only one small one in the center, like N. irrorata (figs. 

 13 to 16). When there is only one small one in the center, the 

 other one is often found attached to the largest chromosome 

 (fig. 16). There are 12 chromosomes in the second division in- 

 cluding the XY dyad lying in the center (figs. 17 to 19). It 

 is assumed that the second small chromosome is always fused 

 with the largest one in this division. No diploid groups have 

 been found in which the number could be determined. In the 

 haploid groups, a largest and usually a next largest chromosome 

 are noticeable and another small one in addition to the one (or 

 two) in the center in the first division. 



IV. THE CHROMOSOMES OF N. GLAUCA 



It has been with particular interest that I have examined the 

 chromosomes of N. glauca because of the previous work of Pan- 

 tel and Sinety ('06) on the same species. Their account of the 

 maturation divisions is so very meager that I feel justified in * 

 giving my own observations in full. I wish to express my sin- 

 cere thanks to Professor Lefroy of The Royal College of Science, 

 London, for his kindness in supplying me with the insects last 

 summer, and to Professor Dobell, in whose laboratory the mate- 

 rial was prepared. 



a. Observations 



In N. glauca there are, in the first division, 13 chromosomes, 

 which appear in polar view as a ring of 12 surrounding one small 

 central one (figs. 20 to 22). The most striking feature of this 

 group, apart from the general arrangement, is the presence of 

 an exceptionally large chromosome which appears as a double 

 or compound chromosome. A similar appearance is sometimes 



