SPERMATOGENESIS IN ASILUS SERICEUS 177 
DISCUSSION 
Asilus sericeus presents the most simple and clear-cut type of 
spermatogenesis thus far found in the Diptera, owing to the 
fact that during the growth period the chromatic threads do 
not spin out and become entangled to such a degree as they do 
in other forms. 
When compared with animals other than Diptera, the most 
outstanding characteristic of the maturation processes in Asilus 
is the apparently continuous association of corresponding chromo- 
somes in pairs. Superficially some of the stages bear a marked 
resemblance to those in various other forms, but on close exami- 
nation it appears that only the later growth period and succeeding 
stages are actually similar in essential features. Previous to 
this there is an underlying difference due to the fact that in 
Asilus the chromosomes, whether condensed or thread-like, 
maintain an intimately paired association from the telophase 
throughout the entire growth period, with the result that the 
usual leptotene stage and the subsequent synaptic process seem 
to be omitted entirely. This is discussed more specifically above. 
Another feature that should be recalled here is the probable 
parallelism between the peculiarities in chromosome behavior 
observed in Asilus and those found in certain plants as recorded 
by Strasburger, Rosenberg, Miiller, Overton, and others (see 
especially Overton, '09). Apparently the peculiarities during 
the maturation stages are in each known case correlated with a 
noticeable paired association of chromosomes in the somatic 
cells, which would again lead one to conclude that the two 
phenomena are causally connected and are both manifestations 
of the same inherent 'tendency toward pairing.' As has been 
remarked previously (Metz, '16, p. 225), the latter seems to be 
an accentuation of the tendency or force that unites correspond- 
ing chromosomes during synapsis in most organisms. It seems 
to differ mainly in that its effects in the cases mentioned are not 
limited to the final germ cells, but are visible in somatic and early 
germ cells as well. What this force is, physicochemically, remains 
as obscure as ever, although there is very strong reason for believ- 
