70 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
condition shown in figure 64, a—h. In this series are found the same 
differentiating characters that have already been described for B, 
except that one member of the large pair of granules at the distal end 
is lacking. In other words, we have to do here with a pair, composed 
of unequal elements, which differs from its homologue in another 
individual, composed of equal elements, by the absence of a definite 
part of one of the components. Examination of all the thirteen 
- individuals demonstrated that eleven of them possessed this second 
\ or unequal type, while only two showed the equal type. 
' If there could have been any doubt about the sequence of events 
in the transformation of a spireme segment into a tetrad and the sub- 
sequent equational division in the case of chromosome-pair A or the 
equal type of B, the behavior of this unequal type of B, as shown in 
figure 64, must certainly make the subject clear. In this instance, 
on account of the difference between the two members, it is possible 
to identify them in such a way that there can be no question as to the 
two planes of longitudinal splitting. The figures have in all cases 
been made with great care with the aid of a camera lucida and are 
faithful reproductions of the conditions seen under the microscope so 
far as they can be represented by the method of reproduction used. 
In the early stages of the transformation of the spireme segments 
into tetrads, the separate chromatids are not distinguishable through- 
out the whole length of the segment. This is due in part to a closer 
association of the chromatids and in part to the fact that one of the 
longitudinal splits becomes more pronounced at one end and the other 
split at the other end of the tetrad. Somewhere between the ends, 
therefore, there is a crossing or apparent chiasma. At the point 
of the crossing the chromatids at first appear to be fused together 
(figs. 63, d and 64, d). Very soon, however, the confusion disappears, 
the chromatids become distinct, and their relationships easily discerni- 
ble, as shown in figures 63, e, and 64, e. In both these cases the wide 
separation at the proximal end has been along the plane of the second- 
ary longitudinal split, and that at the distal end along the plane of 
the primary split. The resulting crossing, or apparent chiasma, is 
a perfectly normal and natural result of these processes and indicates 
nothing in the way of a breaking or recombining of the parts of 
chromatids. 
3. Chromosome-pair C.— Figure 65 (Plate 6) shows one form of 
the third of the three selected chromosome-pairs. In this case the 
two components are very unequal in size, one of them possessing a 
very large, condensed mass, or granule, of chromatin at its distal end, 
