634 
T. H. MORGAN 
Comparisons with the Chaetopterus egg 
Lillie's description of the egg of the annehd, Chaetopterus, and 
of the effects of centrifuging it shows a remarkably close similarity 
between these eggs and those of the nemertean Cerebratulus. 
Lillie's explanation of the formation of the yolk-ring in Chaetop- 
terus applies directly to the similar ring in Cerebratulus, and the 
presence of small basic microsomes in Chaetopterus that collect 
at first at the top of the yolk-ring also finds a counterpart in 
Cerebratulus. The comparison extends even to the presence of a 
superficial ring of yolk-granules imbedded in a denser ectosarc. 
The similarities between the cleavage of nemerteansand annelids 
have been pointed out by Wilson. 
In this connection it is not without interest to find that in both 
forms the polar spindle can be moved from the pole to the center 
of the egg, should the pole be turned outwards on the centrifuge. 
The regularity of the maturation process in Chaetopterus offers 
a better opportunity to study the problem. I have some prepara- 
tions of the eggs of this species that confirm in every respect 
Lillie's interpretation. It is a matter of so much importance for 
subsequent interpretation to make certain that the eggs do not 
orient that I shall consider the evidence in some detail. 
The polar spindles are, after centrifuging, found in the cyto- 
plasm and only rarely in the yolk hemisphere. If we assume that 
the eggs orient on the machine so that the pole near which the 
polar spindle lies moves centrally, we should expect the results to 
be as we find them, i.e.. the polar spindle in the cytoplasmic 
half. But if the spindle has fully formed before the centrifuging 
begins, and has reached the surface, we should still expect to find 
it near the surface and oriented radiall}-, except in so far as it is 
forced in by the presence of the oil cap. In fact, only about half 
of the spindles of the centrifuged eggs lie near the surface and radi- 
ally, the rest lie anywhere in the cytoplasmic hemisphere and 
often near or even in the yolk. This condition would result if in 
all those cases where the spindle lay in the outward hemisphere 
it was displaced by the yolk. This, in fact, is Lillie's inter- 
pretation. It seems to me valid. The only argument opposed to 
