THE PHYSICAL BASIS OF HEREDITY. 183 
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 dongitudinal plane of its former splitting (Fig. 
12, £), and the second division along the ¢ransverse 
plane (Fig. 12, /). 
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 ¢wo chromosomes slightly united end 
to end and split longitudinally. Thus if adcdef ---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 
PPCLOF in the equatorial plate. In the Cyclops 
nucleus of Fig. A the filament has separated into the 
segments ad-cd-ef - - m, each of which has split longi- 
tudinally into = = z, 
subsequently becoming more apparent, gives to each 
. ele “ie, etc. By the first 
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 
etc., and its transverse division, 
.  @ 
tetrad the composition 2 
