STUDIES OX CHROMOSOMES 
65 
d. The Growth-period and M aturation-prophases 
The foregoing facts demonstrate in the clearest manner that 
this individual of M. femoratus differs from all other individuals 
of the genus heretofore examined, with the exception of Mont- 
gomery's material of M. terminalis, in having an odd or unpaired 
idiochromosome (accessory chromosome) which corresponds to 
the larger member of the pair of unequal idiochromosomes found 
in other individuals. They show also that the third small chro- 
mosome is not a small supernumerary of the type found in other 
individuals, and is nothing other than a third 7?i-chromosome. 
This is fully borne out by the growth-period and prophases. As I 
have indicated in earlier papers, the ??2-chromosomes are in general 
characterized during the growth-period by the fact that they re- 
main univalent (there are some exceptions to this) and in most 
cases (of which Metapodius is one) are in a diffuse and hght- 
staining condition. Further, as was first shown by Gross ('04) in 
Syromastes, as a rule they only conjugate to form a bivalent in 
the final prophases of the first division — very often not until the 
spindle is formed and the chromosomes are entering the equator- 
ial plate. Such a late prophase, from a 22-chromosome individual 
of the same species (No. 29) is shown for the sake of comparison 
in fig. 2c, the two separate m-chromosomes appearing above 
and towards the left. Their final conjugation always takes place 
at the center of the group (fig. 2j, 3, e-g). 
In individual Xo. 64, prophases of every stage are shown in 
hundreds of nuclei. In the latest stages, after the nuclear wall 
has broken down, three separate small chromosomes are shown 
(fig. 2a) which may be seen coming together in the final prophases 
to form the small central triad. Figs. 1/^?, n show two earlier 
stages from the same cyst with the last, one of them showing the 
beginning of the spindle-formation, the other an earlier stage 
when the asters are very small and often invisible. Each of these 
shows the three separate small chromosomes, as before. At this 
time all the chromosomes are compact and deeply stained. In 
still earlier stages, at a time when the bivalents are all diffuse and 
appear in the form of lightly staining double crosses, rods, etc., 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 9, NO. 1. 
