SPERMATOGENESIS IN ASILUS SERICEUS 167 
favorable material. It is hoped that the results obtained from 
this will eventually permit of a satisfactory analysis of the pro- 
cesses in the difficult Drosophila material. 
Our survey of the Diptera has not yet revealed any single 
species that is favorable for a study of all stages of spermato- 
genesis, but the species considered here combines more favorable 
features than any other. In this form the seriation of stages 
through the development of the first spermatocyte up to the 
maturation divisions is clear-cut and practically complete. 
Details of certain stages are not depicted with as great clearness 
as thej^ are in some other species (to be considered subsequently), 
but in most respects the material is unusually favorable. 
TECHNIQUE 
Flemming's strong solution has been found most satisfactory 
for fixation and has been used almost exclusively. Heidenhain's 
iron haematoxylin and safranin have been used for chromatic 
stains, with or without counter-stains. A more detailed con- 
sideration of technique has been given in an earlier paper (Metz, 
'16, p. 219). All of the accompanying figures are from material 
fixed in Flemming and stained in iron haematoxylin. 
TOPOGRAPHICAL FEATURES 
The testes in Asilus, like those in most other asilids, are a 
pair of long coiled tubes, each containing thousands of cells. 
The distal end of the testis contains a clearly marked spermato- 
gonial region with a central core of giant nutritive cells. Then 
comes a narrow transition zone in which spermatogonial ana- 
phases and telophases are intermingled with the earliest stages 
of the first spermatocytes. The nuclei here are very small. 
Following this is a broad zone, containing cells, nearly all of 
which are in one stage. From this point the development 
involves a gradual transformation, which may be followed with 
comparative ease through the long growth period extending far 
down the testis. In favorable material all of these stages appear 
in one preparation, or even in one section, and since there is such 
