488 THE AMERICAN NATURALIST. [VOL. XXXIX. 



smaller element. During this mitosis the ten double chromo- 

 somes divide but the single chromosomes remain entire and 

 either pass to one pole or the other or are left out in the forma- 

 tion of the daughter nuclei. The explanation of these conditions 

 must be that ten chromosomes of D. rotundifolia fuse with ten 

 from D. longifolia leaving ten of the latter without mates. 

 Rosenberg's last paper (:O4b) on Drosera describes in consider- 

 able detail the union of chromosomes in pairs in both species of 

 Drosera during sporogenesis. The sporophytic chromosomes 

 which at first are scattered throughout the nucleus in the early 

 prophase of the first mitosis come together in pairs and unite so 

 closely that there is hardly a trace of their dual nature in the 

 resultant larger bivalent chromosomes, which are of course the 

 gametophyte number. Rosenberg is very positive that the pairs 

 of chromosomes are preliminary to a fusion and not the result of 

 a fission of already reduced segments of a spirem thread. 

 Rosenberg believes that the two halves of the bivalent chromo- 

 somes are separated in the first (heterotypic) mitosis and that 

 each splits lengthwise prematurely during the first mitosis in 

 preparation for the second. The fused bivalent chromosomes 

 then appear to divide twice longitudinally but the first division 

 may be only a separation of the two sporophytic chromosomes 

 that entered into the fused pair. 



We shall consider now the conclusions of Berghs and 

 Gregoire of the Carnoy Institute, Louvain, whose publications 

 have appeared practically simultaneously with some of those 

 which we have just discussed. Berghs has published three 

 papers (:O4a, :O4b, : 05) treating of the early history of sporo- 

 genesis in Allium, Lilium, and Convallaria, and concludes from 

 a study of synapsis that the spirem immediately preceding the 

 heterotypic mitosis arises from the close association, side by 

 side, of two delicate threads. These threads are organized pre- 

 vious to and during synapsis and their coming together brings 

 about that contraction of the chromatic material characteristic 

 of synapsis. The threads contain sporophytic chromosomes of 

 the last mitosis in the archesporium. The apparent longitudinal 

 fission of the spirem which precedes the heterotypic mitosis in 

 the spore mother-cell is interpreted as being these two threads 



