204 



THE AMERICAN NATURALIST [Vol. LIV 



ond case this cleft coincides with the apposition-plane of 

 the synaptic mates (i. e., that of the future reduction- 

 division) while the equation-cleft has opened out to form 

 the ring-opening ; and so on in regular alternation. It fol- 

 lows, lastly, that if we disregard for the moment the pos- 

 sibility of an earlier recombination-process, a division 

 that cuts straight through the tetrad, as described alike 

 by Wenrich, Robertson and Janssens, does not in fact di- 

 vide certain rings equationally and others reductionally 

 in regular alternation but divides the whole series in the 

 same way, either equationally or reductionally as the case 

 may be (Figs. 2H,J,3E,F,4cA). 



In order to make clear the contrast between this con- 

 clusion and that of Janssens I have in Fig. 4 A followed 

 his outlines but have indicated the course of the four 

 threads (chromatids) in accordance with the account just 

 given. In Fig. 4 G, on the other hand, the chromatids are 

 shaded black and white in such a manner as to fit with 

 Janssens 's account. A similar comparison is shown for 

 the double-ring tetrads in Figs 2 F, G, which follow Jans- 

 sens 's outlines (2 A, B) as nearly as possible but are dif- 

 ferently shaded ; while 2 H-J shows the double ring and 

 its mode of division in slightly oblique view, so as to 

 show the "chiasma." In these various figures it is at 

 once evident that although a two-strand chiasma or 

 crossing (ch) appears at the junction of every two rings, 

 a straight longitudinal division of such a tetrad (separat- 

 ing black from white) involves on crossing-over, and di- 

 vides every ring reductionally ; i .e., in such a manner as 

 to disjoin the synaptic mates. Here again it is also evi- 

 dent that the multiple ring need involve no twisting of 

 the synaptic mates about one another. It is true that 

 rings of this type, whether single or double, are not infre- 

 quently twisted in their earlier stages, and sometimes in 

 their later— a fact long known and easily verifiable; it is 

 shown unmistakably, for instance, in some of my own 

 slides of Phrynotettix (from material given me by Mc- 

 Clung several years ago). No evidence has yet been pro- 



