WENRICH: spermatogenesis of PHRYNOTETTIX MAGNUS. 123 



characters, are distributed in a linear series along the length of the 

 chromosomes. Then, invoking the aid of the " chiasmatype " theory 

 of Jannsens ('09), they attempt to explain the "cross-overs" by as- 

 suming, first, that when two chromosomes conjugate side by side, 

 they may become twisted around each other, and, secondly, that the 

 later separation is along a plane, which cuts across the threads once 

 for every complete twist. Considering the matter in relation to the 

 tetrad stages, it might be imagined that the two tlireads cross each 

 other, and that at the point of crossing, a weakness of the strands 

 causes them to break and then recombine, forming threads each of 

 which is composed of a part of both the original conjugants. 



Judging from his figures, Janssens founded his theory on conditions 

 similar to those shown in my figure 38, a-d (Plate 3). I am quite 

 sure that the evidence in Phrynotettix does not support the idea that 

 the chromatids break and recombine in any of the postspireme stages. 

 On the contrary, I believe that the chromatids maintain as strict 

 an individuality as I have claimed for the chromosomes themselves. 

 And since these tetrad figures are repeated in so many animals, and 

 even in plants, there would seem to be ground for supposing the be- 

 havior to be similar in all. 



On the basis of this hypothesis, however, Morgan and his pupils 

 have been able to explain the anomalous behavior of the genes which 

 they call the "cross-over" in a very satisfactory way; furthermore, 

 they have been able to use it in connection with the linear-arrangement 

 hypothesis to predict the behavior of any given character, with refer- 

 ence to any other character in the group to which it belongs, provided 

 its behavior in relation to one or two of the characters of the group is 

 known. But there is one point yet to be noted. I have based my 

 criticism of the chiasmatype theory on the conditions as found in 

 spermatogenesis. One of the peculiar facts found in the work on 

 Drosophila is that there is no "crossing-over" in the male. But why 

 should such a phenomenon occur in the female and not in the male? 

 Is it not possible that in the "great growth" period of the oocyte, — 

 where the tetrads become so much more expanded and diffused than 

 in the male, even seeming to disappear entirely in some cases, — the 

 tetrads might suffer some such changes as those suggested by the 

 experimental results? There is also to be considered the often re- 

 peated condition of parasynapsis in Drosophila, as shown by Metz 

 (*14), which might offer greater opportunities for such "cross-overs" 

 to occur than are found in other animals. 



Whatever else may be said of the results of the experiments on 



