EARLY STAGES OF SEGMENTATION OF ECHINUS HYBRIDS. 497 
but these are always so small and faint as to be recognisable 
with difficulty, and in telophase and the 2-cell stage no 
trace of elimination is to be found. 
In the second lot of eggs typical though small vesicles are 
commonly to be found in the first division, and may be seen, 
though with progressively less frequency, in the second, third 
and sometimes the fourth. The vesicles in these eggs are 
in most cases still attached to chromosomes, and are included 
with them in the daughter-plates, and appear to be carried 
into the nucleus. Vesicles are rarelyif ever left on or outside 
the spindle as in the first and second divisions of the 
cross with esculentus. The interpretation of these eggs is 
made difficult by the fact that those in the earlier stages are 
almost all abnormal ; many have multipolar spindles and 
nuclei dividing without division of the cytoplasm, and fre- 
quently there is evidence of polysperm 3 ^ The later stages of 
this batch, however, from the 8-cell stage onwards, appear to 
be normal, and an occasional vesicle may be found in them 
also. We SLig-gest that the behaviour of the eggs in the two 
lots may be reconciled as follows : In the first divisions of the 
cross acutus ? x in i lar i s a small but varying number of 
chromosomes may show a tendency to form vesicles as in 
acutus ? X esculentus d* ; in normall}' developiirg eggs 
the vesicles are usually small and do not become detached 
from the chromosomes, but when the tendency to fonn vesicles 
is present the chromosomes affected probably fail to divide 
and are carried entii-e to one or other pole. In eggs the develop- 
ment of which is delayed and which ai’e abnormal through 
polysperni}’ or other causes, lar-ger and more conspicuous 
vesicles are formed, which, however usually remain attached 
to the chromosomes and so are not eliminated from the 
daughter-nuclei. The non-division of some of the chromo- 
somes is not an easy matter to prove, and in many eggs all 
appear to behave quite normal!}", but that in some eggs one or 
more fail to divide is suggested by three facts : (1) The 
great difference in thickness among the chromosomes in some 
anaphases suggests that some of them have failed to divide. 
