ETHEL NICHOLSON BROWNE. 



idiochromosomes are represented by / and i respectively, the 

 largest chromosome, or macrochromosome by M, the small chrom- 

 osomes, or small autosomes by a. 



Products of the First Division. Products of the Second Division. 

 N.undulata M+gA+I+i+a+a (14) ( M+gA +1 +a +a (13) 



and 



(M+gA+i+a+a (13) 

 either 

 N. insulata M+gA+I+i+a+a (14) "j (Ma+gA+I+a (12) 



or and 



Ma+gA+I+i+a (13)) (.Ma+gA+i+a (12) 



N.irrorala Ma+gA+I+i+a (13) (Ma+gA+I+a (12) 



and 

 (Ma+gA+i+a (12) 



TV. insulata evidently represents a transition stage between TV. 

 undulata with a larger number of chromosomes and TV. irrorata 

 with a smaller number. It is not possible to say which is the 

 primitive condition. We might assume that TV. undulata is the 

 original species, from which, by a process of progressive fusion, 

 have arisen the conditions seen in TV. insulata and TV. irrorata. 

 We might, however, assume that the reverse process has taken 

 place; or, that TV. insulata arose together with one of the other 

 species from the third species. 



The somatic characters give no clue as to which of these inter- 

 pretations should be adopted. We could easily derive the wing 

 coloring of TV. insulata from that of TV. undulata by substituting 

 brown pigment for white, and the wing coloring of TV. irrorata 

 from that of TV. insulata by substituting black pigment for brown, 

 but leaving some of the brown as mottling. But TV. irrorata is 

 intermediate in size between the small species TV. undulata and 

 the large species TV. insulata. Of course the most striking somatic 

 differences are not necessarily those correlated with the fusion 

 or separation of the two chromosomes; these differences may be 

 correlated with other chromosomes which, as I have stated, differ 

 in size in the three species. 



The fact that a transition stage has been found in one species 

 of Notonecta between two other species of the same genus in 

 respect to a fusion and separation of two chromosomes lends 

 support to the views advanced regarding the origin of two or more 

 chromosomes from a single chromosome where the transition 



