£aRLY stages of SEGilENTATIOX OF ECHINUS HYBRIDS. 499 
the number is lower, as is shown in Table IV. The two ends 
of one spindle are bracketed as before. 
The small size of the chromosome groups, with the 
resultant crowding, doubtless makes the numbers in Table IV 
lower than the true figure, and further, Avhen the chromo- 
somes are in very close proximity, it is idways possible for 
two of them, if the fixation is not very perfect, to become so 
united that they appear as one of double thickness. In the 
pure miliar is eggs, although we have sections in which 
34 cbromosomes can be counted with little doubt, the 
majority of counts gave from 31 to 33, and hence we are 
not disposed to lay very great weight on the counts in the 
miliaris hybrids. Nevertheless the frequency with which 
the counts fall below the theoretical number 36, coupled 
with the facts that vesicle-formation on a small scale is not 
infrequently seen in the early stages of division, and that 
in early anaphases swollen chromosomes without visible 
mates at the other pole are also sometimes found, sugg’ests 
that in some eggs at least a reduction in chromosome number 
may be brought about by non-division of one or more 
chromosomes.^ 
It appears, therefore, that in the acutus $ x miliaris 
cross a small number of chromosomes show a varying 
tendency to swell and form vesicles in the early stages of 
the first division; when the tendency is pronounced this 
may cause failure to divide in the metaphase, and they 
become canned to one or other pole. Exceedingly small 
vesicles are sometimes eliminated, but this process is not 
frequent and conspicuous as it is in the cross acutus ? x 
esculentus • In the later divisions the abnormal tendency 
is less marked, and by the third or fourth segmentation 
' A somewhat cursory examination of three batches of eggs of 
acutus ? X mi laris cf, obtained shortly before going to press, 
confirms our account of this hybrid. The eggs, almost all of which 
are fertilised, are chiefly in the later stages of the first division. The 
spindles are like those obtained in 1911 ; most of the figures are normal, 
but a few show very small vesicles, and one case was found of a large 
vesicle eliminated in the telophase. 
