STUDIES ON CHROMOSOMES 101 
One is that suggested by the foregoing phenomena, 7.e., the 
gradual fusion of separate chromosomes into one or the reverse 
process. 
A second mode may be the gradual reduction and final disap- 
pearance of particular chromosome-pairs, as was suggested by 
Paulmier (99), and afterwards by Montgomery and myself, in 
case of the microchromosomes, or ‘m-chromosomes’ of the co- 
reid Hemiptera. In respect to the size of these chromosomes, a 
graded series may be traced from forms in which they are very 
large (as in Protenor) through those where they are of intermediate 
size down to cases where they are very small (as in Archimerus) 
and finally to such a condition as that seen in Pachylis (fig. 
9 j-l) where they are almost as minute as centrioles and may 
almost be regarded as vestigial. Four of these stages are shown 
in fig. 9. In Protenor (a—c) the m-chromosomes are so nearly 
of the same size as the next smallest pair that they often can not 
be positively identified in the spermatogonial groups. In Lepto- 
glossus phyllopus (d-f) they are always recognizable, though not 
much smaller than the next pair. In L. oppositus or L. corcu- 
lus they are a little smaller. In Anasa (the form in which they 
were first discovered by Paulmier) they are of middle size (g-2), 
representing perhaps a fair average of the group. Several other 
genera (e.g., Metapodius) show intermediate stages between this 
condition and that seen in Archimerus (figured in my second 
‘Study,’ and more recently by Morrill) where the m-chromo- 
somes are almost as small as in Pachylis. It is most remarkable 
that throughout this whole series the m-chromosomes exhibit 
essentially the same behavior (Wilson, ’05b, ’06), usually remain- 
ing separate throughout the entire growth-period and only con- 
jugating in the final prophases of the first spermatocyte-division, 
to form a bivalent which with rare exceptions occupies the center 
of the metaphase group; in some forms, also (e.g., Protenor, Aly- 
dus) they show a marked tendency to condense at a much earlier 
period than the other chromosomes. The m-chromosomes of 
Pachylis, excessively minute though they are, exhibit a behavior 
in all respects as constant and characteristic as even the largest 
of the series. In the Lygaeidae they seem to be present in some 
