By happenstance, I stumbled upon a paper---which I would never otherwise have encountered---on Peripatus, with a single reference about a mention by Hickson of Millepora. I am curious. Bercause I am uncertain of the meaning of all of this, I include the small section (about 2 pp) from that paper.
Sheldon, L., 1889. The Maturation of the Ovum in the Cape and New Zealand Species of Peripatus. Studies from the Morphological Laboratory in the University of Cambridge, 4, p.275.
The reference is to:
HICKSON, S. J.-- " On the Sexual Cells and the Early Stages in the
Development of Millepora plicata," Phil. Trans.,' 1888
The following is from Sheldon:
Disappearance of the Germinal Vesicle.
The existence of a stage in the ripening of the ovum in
which no nucleus is visible has now been described in almost
all eggs whose maturation has been at all completely investi
gated. In many of these cases the mode of disappearance has
not been observed , but in most of those in which the process
has been seen it is similar to that which I have described in
Peripatus capensis and P. Balfouri, namely, that the
nucleus loses its round shape, its outline becomes irregular,
and by the loss of its membrane its contents mingle with and
finally become indistinguishable from the protoplasm of the
egg.
Between this stage and that at which the nucleus is
again visible, when it is small and in the spindle stage about
to bud off the second polar body, there is no trace of the ger
minal vesicle in the ovum.
I wish here to draw attention to
the fact that my observations were made on complete series of
sections of well-preserved ova, so that the probability of my
having overlooked it is not great. This mode of disappearance
has been described by Henking (10) in Phalangeden, and by
Stuhlmann (17) in a considerable number of insects. The
loss of the nuclear membrane is also described by Scharff (13)
as occurring in some osseous fishes, although he does not state
that all trace of the nucleus disappears. Lankester (12) also
describes a stage in the ovum of Sepia in which no germinal
vesicle is present .
Blochmann (5) describes in Neritina fluviatilis the loss
of the nuclear membrane, and consequent mingling of the
nucleo- and egg-plasma, but states that the germinal spot
remains, and, breaking up, gives rise to the chromatin of the
nuclear plate. This certainly cannot be the case in the Cape296
LILIAN SHELDON.
species of Peripatus, since the germinal spot disappears before
the germinal vesicle.
Stuhlmann suggests that the disappearance may have some
connection with the presence of yolk in the egg, since he has
observed it in all insect eggs in which yolk is present in large
quantities, but not in Aphides and Cecidomya, in which yolk
is absent. Its disappearance in Peripatus capensis , how
ever, in which no yolk is present, goes against this theory,
unless it is to be regarded as a survival from the time when yolk
was present in the ovum .
On the other hand, it is supported
>
by the observations of Will (20) and Scharff (13) , who state
that some of the yolk is derived from the germinal vesicle,
and by the phenomena which I have described in P. novæ
zealandiæ ; but these points will be discussed later under
the Formation of the Yolk .
Stuhlmann ( 17) has not observed the disappearance of the
germinal vesicle in P. Edwardsii , but this may be due to the
incompleteness of his researches in this species .
The double disappearance of the germinal vesicle in P. novæ
zealandiæ is, so far as I know, unparalleled, and I am unable
to offer any explanation of it. The first disappearance seems
to be that which is homologous with that which commonly
occurs in ova, and its mode of disappearance will be discussed
later on. The details of the second disappearance are quite
unknown, and at first sight it would appear that the nucleus
shown in the ovum in fig . 30 was the first segmentation
nucleus ; but that this is not the case seems certain from the
fact that a large proportion of the unsegmented ova which were
found in the uterus were without any nucleus, and it is not
easy to conclude that this condition was abnormal since the
ova were found in several different parents, and were preserved
in several ways . The only case in which, so far as I know , a
double disappearance has been described, is that of Millepora,
in which Mr. Hickson ( 11) states that the nucleus is dispersed
before and after the formation of the polar bodies ; but since
in P. novæ - zealandiæ I have observed no polar bodies, it
is not possible to compare the two cases .
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