176 CHARLES W. METZ AND JOSE F. NONIDEZ 
THE NUCLEOLUS 
The history of the nucleolar structures has not been studied 
in detail, but the nucleolus is so prominent during the growth 
period that a study of the chromosomes must necessarily reveal 
the main features of the nucleolar behavior. It is probable 
that the chromatic part of the nucleolar complex persists from 
the final spermatogonial anaphase in the form of a pair of chromo- 
somes, but whether the achromatic portion arises from this or 
originates independently we are unable to state. The two are 
united from stage b (fig. 11) throughout the remainder of the 
growth period. The chromatic portion may be followed directly 
to the first spermatocyte metaphase where it becomes one of the 
five bivalent chromosomes. During much of the growth period 
the nucleolus is plainly compound (fig. 14), being composed of a 
large, oval achromatic portion and a smaller dense chromatic 
portion to which is attached a chromatic thread or finger-like 
projection. The latter is very characteristic and persistent 
throughout the growth period. The achromatic portion seems 
to diminish gradually during the later stages, and cannot be 
detected with certainty in late prophase. However, the degree 
of extraction of the haematoxylin has much to do with the appear- 
ance of the structure, and it is difficult to say just what becomes 
of the achromatic portion. 
The chromatic portion is presumably the sex chromosome 
pair. At first sight the finger-like process suggests the presence 
of an unequal XY pair, but this asymmetry seems to disappear 
in metaphase and we are unable to verify the point. Likewise, 
the spermatogonial divisions do not reveal any such inequality 
in any chromosome pair. It seems more probable that the 
finger-like process is due to a difference in the degree of conden- 
sation of the two chromosomes, such, e.g., as that shown by the 
XY chromosomes of Enchenopa binotata (Kornhauser, '14). 
In another species of Asilus it is practically certain that the 
chromosomes involved in the nucleolar complex are the sex 
chromosomes, and it may be inferred that the same is true in 
A. sericeus. This is in agreement with the observations of 
Stevens ('08 b), who found the sex chromosomes condensed 
during the growth period in several species of flies. 
v^-. 
