STUDIES ON CHROMOSOMES 383 



to arise by a drawing out of the central region of each longitudinal 

 half from the synaptic point to form the 'lateral arms' of the cross. 

 The ring-form was supposed to arise by a secondary bending 

 around of the two principal arms until the free ends united; the 

 F-forms by a sharp flexure of the bivalent at the synaptic point 

 (cf. Paulmier, '98, '99). On the whole, however, it seems to me 

 that the evidence points more strongly to the opposite interpre- 

 tation, first clearly worked out by the Schreiners ('06) for the 

 closely similar figures seen in Tomopteris. According to this, 

 the original condition is that of two parallel threads or rods, in 

 parasynaptic association, each of which sooner or later undergoes 

 longitudinal fission. The rings are described as arising by an 

 opening apart of the two rods along their middle portions while 

 remaining attached by both ends; the F's by an opening apart 

 from one end, while remaining attached at the other; and from 

 the latter, by complete opening out of the two limbs until they 

 are in a straight line, arise the tetrad-rods. From the latter the 

 crosses are readily derived by drawing out of the lateral arms in the 

 manner assumed by Paulmier. 



Though all this is somewhat hypothetical as applied to these 

 insects, I consider it the more probable view for several reasons. 

 The first of these is the evidence that the lateral halves of the dip- 

 lotene-threads begin to separate already at the end of Stage / 

 as the nuclei are passing into the confused stage, and the correlated 

 fact that in the initial prophases the bivalents are seen drawing 

 together out of more or less widely separated single threads. A 

 second is the prevalence of the double F-figures in the early pro- 

 phases, and their gradual disappearance as the prophases advance. 

 It is evident that these F-figures are opening apart in these stages, 

 not closing up. Elongated F's with their Hmbs often nearly 

 parallel (figs. 107, 108, photo. 17) are commonly seen in smear- 

 preparations of these stages, and in slightly later ones all inter- 

 mediate stages connect them with the tetrad-rods or double 

 crosses (figs. 109 to 114, photo. 18). These facts are quite inde- 

 pendent of any particular conception of synapsis, but they seem 

 to fit best with the view that the original type is a longitudinally 

 double rod following parasynapsis, as maintained by the Schrei- 



