400 MITCHEL CARROLL 
13) will show that the extra elements are those which have been 
placed in the column marked ‘S.’ The extra dyads (S in figs. 6 
to 10, 24 to 26, 32, 74 to 81, 85, 91 to 98, 105, 106, 136) are 
always easily identified by their size, form, and position on the | 
spindle. The supernumerary tetrad (S in figs. 1 to 5, 27 to 30, 
57 to 61, 72, 73, 82 to 86, 120 to 130) is readily distinguished by 
its size and form from all other chromosomes except no. 3, and I 
believe it can ordinarily be distinguished from the latter. 
In the spermatogonia, and usually in second spermatocytes, we 
have only the relative size of the extra elements as a criterion. 
Hence identification of the supernumeraries in the complexes of 
these two cell generations is often not absolute. 
The extra dyads in the column labeled ‘S’ in plates 1 (figs. 6 
to 10), 3 (figs. 24 to 26), 6 (figs. 52 to 55), 8 (figs. 74 to 81), 9 
(figs. 85, 91, 92), 11 (figs. 105, 106), and 14 (fig. 136) are clearly 
homologues in size and form. The supernumerary tetrad (S in 
figs: 1 to.5; 27 to 30; .52 to 61,282: to-86; 120: fo. 128) ieee 
evidently composed of two of these homologous dyads. 
It is true that slight differences in the apparent size of the 
silhouettes of both the extra dyads and extra tetrads in different 
complexes are discernible. But these variations are not constant 
even for the cyst and are no greater than those seen in other 
chromosomes, as, for instance, no. 3 or no. 4 (the accessory). 
Apparent differences in the size of homologous elements may be 
due to foreshortening or varying relations of the chromatids 
within the chromosomes (McClung, ’14; Wenrich, ’16,’17). Real 
differences in the size of genetically related elements can and do 
exist as in the tetrads with unequal homologues reported by 
Carothers (’13, ’17), Voinov (’14), Wenrich (’16), and Robert- 
son (715). But such slight differences as may actually exist 
between these homologous supernumeraries in Camnula are not 
due to the loss of a specific part of the chromosome as in the 
above unequal pairs. They seem to be the result, rather, of vary- 
ing degrees of condensation of the chromatin. For instance, the 
extra dyad in figure 79, plate 8, appears somewhat larger than . 
the other supernumeraries of like valence for the reason that it 
has a constricted ‘neck’ and a knob on one end. This same 
