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THE AMEBIC AN NATURALIST [Vol. LIV 



owing to the appearance of a second cleft at right angles 

 to the first. The evidence is nearly or quite conclusive 

 that one of these clefts coincides with the original plane 

 of synapsis or side-by-side apposition of the synaptic 

 mates (also the plane of the future reduction-division) 

 while the other is the equation-plane along which each 

 synaptic mate is longitudinally split. 2 In any case it is 

 generally agreed that single rings arise by the separa- 

 tion and opening out of these threads along one of the 

 clefts (generally believed to be the synaptic, as in Fig. 

 5 1), their ends remaining united, while the second cleft 

 remains as the longitudinal cleft of the ring and repre- 

 sents the plane of the equation-division. The lateral 

 arms of these rings arise, as shown in the figures (5 I, 

 B, C) by separation and divergence of the free ends for 

 a certain distance along the second (equational) cleft, 

 thus finally giving the appearance of a double cross at 

 this part of the ring (5 I, D). 



Double rings, coupled together (Fig. 5 II) arise when 

 the rods separate along different planes in two adjoin- 

 ing regions, the opening of one ring representing the 

 expanded synaptic cleft (appositional or reductional) 

 that of the other the equation-cleft. Such rings are of 

 course at right angles to each other ; and as the diagram 

 shows (Fig. 5 II, B, D) when these tetrads are viewed 

 obliquely they seem to show at certain points crossed 

 threads or chiasmas (c~h.) in which two threads cross 

 over from opposite sides. 3 It is of the first importance, 

 however, to bear in mind the fact that such figures are 

 shown in fore-shortened view. They are an attempt to 

 represent in two dimensions a figure which actually is in 

 three dimensions. Such tetrads can not adequately be 

 visualized until modeled in clay or by means of wires, so 

 as to be seen in three dimensions. When the models are 

 obliquely viewed they seem indeed to show at each node 



« This interpretation disregards the possibility (which I think is a proba- 



tween the synaptic mates may already have occurred in the quadripartite 

 rod; but for the moment we may leave this out of account. 



3 This is clearly shown in McClung's photograph, Fig. 122 ('14). 



