Oct.. I92I] EMERSON ABERRANT CHROMOSOME BEHAVIOR 
well be assumed, therefore, that non-disjunction within this one group of 
chromosomes occurs three times as frequently as it is visibly manifested in 
such material. 
Moreover, there is no satisfactory evidence that one group of homologous 
chromosomes is involved more frequently than any one of the other nine 
groups. It may be supposed, therefore, that non-disjunction occurs on 
the average 30 times for every aberrant seed observed in material hetero- 
zygous for a single factor pair. Certainly non-disjunction — if such be the 
cause of aberrant endosperm — ^is not limited to the C-I-Sh-Wx chromosome. 
Aberrant endosperm has been ofl served to involve the additional aleurone 
and endosperm factors A, R, Pr, F, and Sti, all of which are inherited inde- 
pendently of the C-I-Sh-Wx group, and all of which, except possibly R and 
Pr, are inherited independently of one another. Since, therefore, aberrant 
endosperm has been observed to involve not less than five and perhaps six 
linkage groups, aberrant endosperm behavior is assumed to have occurred 
in at least the same number of groups of homologous chromosomes and there 
is no reason to suppose that it is not common to all ten groups. 
From tables 1-3 of this paper, it is seen that 150 aberrant seeds were 
observed with an approximate total of 57,830 normal seeds. Of these 150 
seeds, 13 involved sugary endosperm, a character that might easily be 
overlooked except when the aberrant spot is large. Since, moreover, the 
material involving sugary endosperm had to do also with an aleurone-color 
factor, the 13 seeds must be omitted if we are to deal with a single factor or 
linked-factor group, in other words, with a single chromosome, at a time. 
The observed ratio, when only one factor is involved, is 57830 : 137, or 
approximately one aberrant case in every 423 seeds. If now it be assumed 
that non-disjunction occurs thirty times as frequently as aberrant seeds 
in such material, non-disjunction should occur on the average once in about 
14 seeds. In more than half of the aberrant seeds reported in this paper (77 
out of 150) the aberrant part included approximately one sixth or more of 
the surface area of the seed and in about one twelfth of them it included 
more than one half of the seed. Consequently, non-disjunction must occur, 
if at all, fairly early in endosperm development in a considerable percentage 
of cases. It would seem worth while, therefore, for cytologists to search for 
it at least in the early divisions of the endosperm nucleus. 
An observation noted earlier in this paper suggests that irregularities 
besides non-disjunction may occur in endosperm development. It was 
noted that 8 out of 65 aberrant seeds involving C c and Wx zvx were mottled, 
exhibiting numerous small spots of colorless aleurone instead of a single 
spot. One of these mottled seeds was so immature that the endosperm 
texture could not be determined, but in the other seven the colorless spots 
were underlaid by waxy endosperm (fig. i, L), this association indicating 
definitely aberrant chromosome behavior. It does not seem likely that 
non-disjunction would occur repeatedly in the development of a single 
