No. 632] CHIASMATYPE AND CROSSING OVER 197 



Eing-tetrads may be single, or may consist of two or 

 more rings joined together in such a manner as to be suc- 

 cessively at right angles to one another, as is schemat- 

 ically shown in Janssens's diagram, here reproduced in 

 Figs. 1 and 2 A-C. Single rings of the type here in ques- 

 tion (Fig. 3A,B) were I think first clearly described and 

 figured in my laboratory by Paulmier ( '98) in Hemiptera, 

 though he did not correctly make out their mode of origin. 

 Similar rings were subsequently studied in many other 

 animals, e. g., in Orthoptera by McClung, Sutton, Gra- 

 nata and others, in urodeles by Janssens, and in anne- 

 lids by the Schreiners, Foot and Strobell and others. 

 More recently they have been carefully examined by a 

 number of observers, in particular by McClung ('14), 

 Robertson ('16), and Wenrich ('16, '17). The single 

 ring-tetrad (Fig. 3 A, B) consists of a more or less open 

 ring, split lengthwise into two closely apposed halves and 

 cut crosswise at opposite points by two sutures which 

 divide the ring into two semicircular half-rings. The 

 latter are now regarded by practically all observers 

 (Janssens included) as the synaptic mates, joined by 

 their ends but elsewhere widely separated so as to lie on 

 opposite sides of the ring-opening, and each longitudi- 

 nally split. The longitudinal cleft lies therefore in the 

 plane of the future equation-division, the cross-sutures 

 in that of the reduction-division. At one of the cross- 

 sutures, less often at both, the longitudinal halves of 

 both synaptic mates are commonly drawn out at right 

 angles to the ring (in the manner made clear by Fig. 3 

 A, B) thus forming two lateral arms, each longitudinally 

 double, so that this part of the ring, as seen in face view, 

 offers the figure of a double cross. If the ring be sup- 

 posed to break in two at the opposite suture and the half- 

 rings to straighten out completely it would become a sim- 

 ple double cross-figure with two short arms and two long 

 (Fig. 3 C or 5 III). If, on the other hand, the lateral 

 arms of a closed ring be supposed to elongate still more 

 and to bend away from the original ring until they meet, 



