534 P. W. WHITING 



figures, pair b is the small pair. Figure 51 shows that the 

 chromatids of pair a are changing partners at both ends, thus 

 forming a ring with a cross at each side. The difference in 

 length of the transverse arms of the cross is a matter of per- 

 spective. Figure 52 shows the three pairs, each with a change 

 of partners at one end. In a the split of the ends is in the 

 plane of the paper and the lower point of union is bent down so 

 that the chromatids uniting there appear shorter on account 

 of perspective. 



In figure 53 each of the three pairs has formed a ring, with a 

 change of partners at each end. This is a typical form of pro- 

 phase tetrad. The tetrads have been displaced for clearness in 

 drawing. The direction and extent of displacement is repre- 

 sented by the direction and the length of the arrows. 



In figure 54 each of the three pairs has formed a cross by a 

 change of partners comparable to text figures 5 and 6. This is 

 also a typical form of prophase tetrad. Tetrads of either the 

 ring or the cross variety may thus be formed by any one of the 

 three pairs. 



In figure 55 a and b are tetrad crosses, while c has the members 

 still separated. In figure 56 a and b are rings, the latter, however, 

 incomplete at the lower focus. Tetrad c is a cross seen from 

 side view. 



The change from late prophase to metaphase and anaphase 

 is passed through by each tetrad independently of the others. 

 In figure 57 tetrads b and c are in early anaphase while a is still 

 in prophase. The prophase tetrads are thicker and less clean- 

 cut than those in metaphase. This is probably due to relative 

 degree of condensation. 



In figure 58 b and c are in late metaphase, showing clearly 

 their fibre attachments. Tetrad a is a thick, irregular prophase 

 cross without apparent fiber attachments. In figure 59 the small 

 tetrad b is still in prophase, but the outlines are more clean-cut, 

 showing a certain degree of condensation. In figure 60 the out- 

 lines of the cross-shaped tetrad c are smooth and a fiber attaches 

 to one of its arms. Tetrad a has separated irregularly so that 

 a part of the upper member is still attached to the member 

 going to the lower pole. 



