STUDIES ON CHROMOSOMES 101 



One is that suggested by the foregoing phenomena, i.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-i) , 

 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, '056, '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 



