CHROMOSOME STUDIES 209 



(figs. 156a, 156b), after condensing, enters the metaphase plate 

 in this extended condition. 



Figure 156c is typical of the so-called cross form. The four 

 parts are visible. The proximal knobs (x, x) appear similar 

 to those of figure 156b. The arms of the cross extending along 

 the axis of the primary split (/, I) may be bent about until their 

 ends are almost in contact, as in no. 6, figure 150b, or even in 

 contact, giving a ring-shaped chromosome, as in nos. 4, 6, and 

 9, figure 155. 



The condition shown in figure 156d may be produced from 

 that of figure 156c by a lengthening of the arms along the axis 

 of the primary split and by so bringing their distal ends into 

 contact that the whole chromosome forms a ring. With the 

 fusion of the distal ends to form the ring, the components in 

 that region begin to separate along the primary plane of division. 

 At the proximal part of the chromosome, where this stage has 

 been reached, the primary split (/, /) prevails; in the middle 

 part, the secondary split (//, //) prevails; and at the distal end, 

 the primary split again prevails, giving two knobs slightly smaller 

 than the proximal knobs (x, x). By decreasing the extent of 

 the secondary split (thus lessening the diameter of the ring) 

 and increasing the primary split (thus increasing the length of 

 both proximal-knob [x, x] and distal-knob ends), we get as a result 

 the conditions of figures 156e, 155 no. 10, and 152a. Starting 

 again with the ring-like stage, by increasing the length of the 

 distal-knob ends and then bringing these knobs into contact 

 again at the extreme distal ends (thus forming a second ring 

 whose plane is perpendicular to that of the first ring), and by 

 then decreasing the length of the proximal-knob ends (x, x) 

 and the extent of the first ring, we obtain a condition like that 

 of figure 156f. I do not say that a continuous change takes 

 place along the axes of the 'splits' so as to produce all these differ- 

 ent forms in one chromosome, but rather that as the conjugated 

 chromosome — split in two longitudinal planes, the primary and 

 secondary (7, / and II, II), at right angles to each other — comes 

 out of the synapsis period, its four parts present one or the other 

 of these conditions, separating most along the primary or second- 



