THE PHYSICAL BASIS OF HEREDITY. ^3 



clearly that each chromatin segment has divided both 

 longitudinally and transversely, its parts shortening and 

 arranging themselves in the tetrad formation of Fig. 12, 

 D. The first division following separates the tetrad 

 along the longitudinal plane of its former splitting (Fig. 

 12, ), and the second division along the transverse 

 plane (Fig. 12, F}. 



In Cyclops then the tetrads are formed by the chro- 

 matin thread of the resting nucleus breaking up into one 

 half the usual number of segments, and each of these in 

 turn dividing longitudinally and transversely. A tetrad 

 here is made up of two chromosomes slightly united end 

 to end and split longitudinally. Thus \iabcdef n rep- 

 resent the unsegmented filament of the resting nucleus, 

 a-b-c-d-e-f would show its breaking up into the normal 

 number of chromosomes which split lengthwise, forming 



a b c d e f 



, -r, , -j, , ^r in the equatorial plate. In the Cyclops 



nucleus of Fig. A the filament has separated into the 

 segments ab-cd-ef - - - n, each of which has split longi- 

 tudinally into , -3, -~.j etc., and its transverse division, 

 subsequently becoming more apparent, gives to each 



tetrad the composition , , K, etc. By the first 



a\b c\d e\f 



division, in the longitudinal plane, each daughter cell 

 receives a half of each chromosome ; in the second, 

 however, in the vertical plane, this is not the case, as 

 can be readily seen. This is clearly a qualitative di- 

 vision, and the daughter cells receive unlike chromo- 

 somes. This forms the " reducing division " in Weis- 

 mann's sense, and as such is a most beautiful demon- 

 stration of his postulated reduction of the ancestral 

 plasm. 



In Ascaris, however, the evidence is just as clear 



