28 ETHEL NICHOLSON BROWNE. 



EXPLANATION OF PLATE II. 



Notonecta irrorata. Figs. 12, 13, metaphase of first division showing 13 chromo- 

 somes, one small one in the center. Fig. 14, initial anaphase of first division, 

 side view, one small pair in the center. Fig. 15, metaphase of second division 

 showing 12 chromosomes, large idiochromosome in the center. Fig. 16, the same, 

 but both idiochromosomes showing in the center. Fig. 17, initial anaphase of 

 second division, side view, idiochromosome-pair in the center. Fig. 18, the same, 

 but idiochromosomes on different fibers. Fig. 19, A and B, sister anaphase groups 

 of second division, from the same spindle. Fig. 20, spermatogonial group showing 

 24 chromosomes. 



