86 ETHEL NICHOLSON BROWNE 



chromosomes; it seems, on the contrary, altogether reasonable 

 to suppose that the essential chromosome elements retain their 

 identity throughout the entire process. 



V. PROPHASES 

 A. General description 



1. Notonecta insulata. It is of interest to trace the history of 

 the chromatin from the dissolution of the karyosphere until the 

 formation of the definitive chromosomes. After the irregular 

 masses of chromatin have left the plasmasome, they pass from the 

 interior of the cell to the nuclear membrane; and here the 



. chromosomes pass through a diffuse stage before assuming their 

 final form. At first they appear on the nuclear wall as longitudin- 

 ally split rods, long, thin and somewhat curved (fig. 78); the 

 rods are apparently made up of a linear series of granules 

 (chromomeres) which give them an irregular contour. The usual 

 prophase figures, rings, crosses, etc., are formed from the longitu- 

 dinally split threads (fig. 79) ; they will be described in detail later. 

 By a process of condensation are formed the definitive chromo- 

 somes which are typically dyad-like in appearance; their tetrad 

 nature cannot be made out unless they lie in a favorable position 

 and are very critically observed (fig. 80). During these stages, 

 the chromosomes have remained close against the nuclear mem- 

 brane, and it is from this position that they are drawn on to the 

 spindle in the late prophase. In figure 81 they are seen irregularly^ 

 arranged on the spindle, prior to their final grouping around the 

 equator. Attention may be called to the fact that frequently in 

 the late prophase, the small chromosome is found attached to the 

 large one, forming the compound chromosome, to which reference 

 has been made in an earlier part of the paper (figs. 80, 81). 



2. Notonecta irrorata. The history for this species is practically 

 the same. The thin longitudinally split rods (fig. 82) on the nu- 



»clear wall give rise to rings, crosses, etc. (fig. 83). While still on 

 the nuclear wall, they condense into the definitive chromosomes, 

 which later become irregularly arranged on the spindle (fig. 84). 



