On Morphological Difference of the Chromosomes of Ascaris megaloceph. 73 
tbe second polar spiudles shown on PI. 7, Figs. 53—65, oue may 
uote specially the two elements of the smaller dyad shown in Figs. 54, 
55, 59, 61, 65. Thns the egg retains sonietimes the smaller and so- 
metimes the larger element of this smaller dyad, which retained ele- 
ment would have sometimes the volume of the smaller sperm chro- 
mosome and have sometimes a diö'erent volume. It is at least re- 
markable, and speaks for persistiug chromosome difference, that it 
is ouly the smaller dyad of the second spindle that sometimes shows 
marked size difference of its elements, in conjimction with the fact 
that the smaller chromosomes of the cleavage differ more from each 
other than do the larger. 
Here I wish to correct an error of observation in an earlier 
paper (1906). I then reasoned from the examination of the sperma- 
togenesis of various Hemiptera.- »When I first discovered the con- 
stancy of such chromosome pairs, I concluded that the two compo- 
nents of each pair were exactly equal in form and volume, and so 
have the others who followed me. In the present paper I have 
given especial attention to this point, and now find good evidence 
that the components of each pair are probably constantly slightly 
different from each other in volume.“ The evidence was based part- 
ly on the difi'erences between corresponding diplosomes, partly on 
the unmodified chromosomes of Corizus alternatus and Harmostes 
reflexulus. As an example I reproduce in Fig. 75, PI. 7, the sper- 
matogonial chromosome plate of Corizus that was given in Fig. 107 
of my paper of 1906. Mo is the monosome, Di and di the diplo- 
somes, and A, a-E, e the pairs of ordinary chromosomes. Of the 
latter the pair A, a is of interest, the largest elements of all, for in 
this cell as well as in the only two other clear cases these two chro- 
mosomes are slightly different in volume and markedly in form. The 
observation that calls for correction is that on Harmostes; Fig. 74, 
PI. 2, is a redrawiug of Fig. 94 of the earlier paper. Then I had 
misiuterpreted the chromosome pairs, and so had been led to assume 
differences in volume between the two elements of a pair. But the 
relettering here given shows that the two of each pair are relatively 
equal, and that the monosome is the longest. Harmostes is therefore 
not a case showing morphological difference within the same chro- 
mosome pair. 
While so far as present evidence goes the two chromosomes of 
a particular pair seem to be as a rule indistinguishable from each 
other, this is not always so. The smaller pair of Ascaris sometimes 
