MALE GERM CELLS IN NOTONECTA 



71 



can actually be observed in N. insulata, it seems reasonable to 

 attribute its absence in N. irrorata to its permanent association 

 with the largest chromosome. Representing the large chromo- 

 some, or macrochromosome by M, the two small autosomes by 

 a, the unequal chromosomes by A", Y, and the larger autosomes by 

 A, we may schematize the results as follows i^ 



The scheme shows the intermediate condition of N. insulata 

 between X. undulata with a larger number of chromosomes and 

 N. irrorata with a smaller number. The large and small 

 chromosomes in N. undulata are always separate, in N. insulata 

 sometimes separate and sometimes associated, and in N. irror- 

 ata are presumably always associated. This may represent a 

 progressive (or regressive) series, or the three forms may represent 

 different modifications of a single original type. 



The somatic characters do not afford decisive evidence concern- 

 ing these three possibilities, although the wing color fits in with 

 the view that N. insulata is an intermediate species. By sub- 

 stituting brown pigment for the white of N. undulata, the wing 

 coloring and pattern of N. insulata is obtained; further, by sub- 

 stituting for this brown pigment, black, but leaving some of the 

 brown as mottling, the wing pattern of N. irrorata is obtained. 

 On the other hand, N. irrorata is intermediate in size between the 

 other two, and N. undulata is intermediate in respect to the distance 



1 This scheme is identical with that published in my preliminary paper ('10) 

 except that X, Y have been substituted for /, i. 



