70 bulletin: museum of comparative zoology. 



condition shown in figure 64, a-h. In this series are found the same 

 differentiating characters that have ah-eady 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 beha^•ior 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 chromati<^ls anfl 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 68, r, and 64, r. In botli 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 wa^^ 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, 



