178 CHARLES W. METZ AND JOSE F. NONIDEZ 
ing that it is due to a likeness in constitution of corresponding 
chromosomes. 
Regarding the genetical question of 'crossing over,' our obser- 
vations afford only negative evidence. Since no leptotene 
threads have been observed and nothing hke a typical synaptic 
stage has been identified, there is little indication of any process 
that might bring about crossing over during the early stages. 
It is difficult to determine just what takes place in stage a, but 
it should be recalled that this stage appears to be like the sperma- 
togonial resting stage, and that as far as cytological evidence 
goes there is no more reason for expecting it in the former than 
in the latter. 
In subsequent stages there is some evidence of chromosome 
twisting, but more often the threads lie side by side without 
twisting, and when they do overlap there is no evidence of break- 
ing. On the whole, then, what evidence there is would argue 
against the probability of crossing over in the males of Asiliis, 
which agrees with the genetical results in Drosophila, where 
crossing over is found only in the female. But this question, 
from the cytological standpoint, is only in the speculative stage, 
and will probably remain there at least until further studies are 
completed, particularly studies on oogenesis. 
In this connection a word should be said regarding the degree 
to which the above description may be considered typical of the 
Diptera. One other species of Asilus (A. notatus) has been 
studied fully and shows certain noteworthy deviations from the 
above account. These may be summarized briefly as follows: 
In stage a following the final spermatogonial telophase, the 
chromatin stains more deeply than in A. sericeus and gives even 
clearer evidence of remaining relatively condensed, i.e., not 
spinning out into threads. Stage a is very brief and is succeeded 
immediately by stage h, in which the chromatin is likewise more 
dense than that in the corresponding stage of A. sericeus. It 
is in the form of short, thick, bivalent prochromosome-like 
bodies, the dual nature of which is very plain. These show a 
more marked superficial resemblance to the bodies of stage h 
in the Hemiptera than do those of A. sericeus. 
