168 CHARLES W. METZ AND JOSE F. NONIDEZ 
a large number of cells available for study, even the intermediate 
transition stages, often so difficult to obtain, are nearly all in 
serial order. The only exceptions to this rule are found in the 
stages immediately following the last spermatogonial anaphases. 
Here development proceeds with great rapidity, and the telo- 
phase and associated stages are more or less intermingled. But 
in spite of this it is possible to obtain a clear conception of what 
takes place, for the immediately succeeding stage (5) is perfectly 
clear, and with its aid the other figures may be pieced together. 
The details of these processes will be given below. 
SPERMATOGONIA 
The spermatogonia are abundantly represented in our material 
and are of ample size for study. Those of the last generation or 
two are noticeably smaller than the earlier ones and have the 
chromosomes more closely aggregated in metaphase, but other- 
wise are not appreciably different from the rest. In a previous 
paper (Metz, '16) the general peculiarities of chromosome behav- 
ior in spermatogonia and other diploid cells of flies have been 
described in detail. For this reason the spermatogonial stages 
will be passed over briefly here. It is important, however, to 
keep in mind the fact that the close association of homologous 
chromosomes, especially in the resting stages and prophases, 
makes the pairs of chromosomes simulate single chromosomes 
of other animals. 
In spermatogonial metaphases of A. sericeus there are ten 
chromosomes, arranged in five pairs (figs. 3 and 4), the smallest 
of which is probably the sex chromosome pair, although there 
is no evident dimorphism to distinguish it. Occasionally the 
arrangement of one or two pairs is disturbed, but normally they 
all show the paired association just as in other Diptera (Metz, 
'16). In anaphase the chromosomes pass to the poles in this 
paired condition (fig. 5). Their behavior during the telophase 
will be discussed below. 
During most of the resting stage the chromatin is diffuse and 
stains so faintly that its behavior cannot be observed satis- 
factorily. In prophase it becomes aggregated, and each aggre- 
