An obscure mention of Millepora work by Hickson: "double disappearance" of germinal vesicle (I think)

 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 .

 

 

Colored figures from Hickson

I am cautious about Hickson for a couple of reasons.  But he did so much!  I should point out a couple of things:

 Hickson's description and drawing of the medusa of Millepora does not reflect the actual appearance of the medusae I have seen. It appears to be of a female medusa that is in the process of expelling it's cargo of ova.  That illustration has been borne by many of not most Invertebrate Textbooks, as a minor revision of that from Moseley's work, probably drawn by J.J. Wilde, Challenger artist.  All of that is another story.

Another extreme example of Hickson's lack of caution was his synonymizing all Millepora species as one: M. alcicornis.  

In my mind his work on the reproductive physiology and anatomy of Millepora spp. suspect.  One of the volumes replete with illustrations, needs to be studied: I think he got reproduction COMPLETELY wrong for Millepora spp.  To be fair, he may have corrected this error in a later publication.

 I still think that his abundant visual expositions of Histology of Millepora are worth study, especially for one like myself who is just learning.  Caution is the watchword.

 These images were captured from Google Books's of

Hickson, S.J., 1891. The Medusae of Millepora Murrayi and the Gonophores of Allopora and Distichopora,"'Quart. Journ. Micr. Sci.,'xxxii.

 

This seems to be a unique copy, with colored figures.  Possibly colored by hand?

 

Calicoblast layer under dactylozoid

 

Clustering proliferation of dividing symbiotic dinoflagellates in Millepora sp. medusa/egg

 I will post this image for now, and get back to it.  These symbionts infect or are drawn into the medusa/egg before liberation from the mother colony.  As far as I have seen, all medusae of a given colony are either male or female.  The larva is pre-charged with it's symbionts.   The order and nature of events during the infection process are of great interest.  

From a stack of 9 images.

Annotated image to demonstrate the dividing cells with two nuclei.  Other cells, more visible in the image above, are near enough that perhaps they are just recently divided.

Ongoing imaging explorations

Millepora platyphylla: interface between medusa and ampulla. 

Phase contrast

 

I was exploring phase contrast with a new--to me--used Zeiss Plan Ph2 40/0.65 160/0.17 objective.  This slide was stained using Cason Trichrome stain.  

This shows a part of a medusa, with scarcely developed ova.  I think most of will contribute themselves to the growth of the three, or possibly, sometimes, four ova that eventually develop.  These may be called nurse cells?  

A meshwork of vacuolated material---lipid droplets---is being traversed by some zooxanthellae.  The eggs of Millepora spp. are hosts to symbiotic dinoflagellates (zooxanthellae) before their release and fertilization.  This is part of the process.

 

 The ampulla is a hollow vessel that holds the medusa as it develops.  The tissue outside the medusa is apparently related to the ampulla.  Intense yellow indicates calicoblastic tissue.  I am interested that in this image the yellow resolves into green and yellow particles; I wonder, does this point to two separate structures involved in producing---and in this species sometimes dissolving?---Calcium Carbonate.  To my knowledge the structural changes involved in generation of ampullae, in conjunction with growth of the medusae and development of the gametes in both male and female medusae has not been worked out in its entirety.  The reproductive events, per se, have been studied closely.

 

 

 

Photographs: structure of Millepora platyphylla skeleton

The Tabulae are apparent in the following photographs of Millepora platyphylla Skeletons

 

. 

I was attracted to the study of Millepora platyphylla because of their hard parts, their skeleta.  My overriding interest was in reproductive timing.  The Milleporid  Hydrozoan corals had me from where I learned that their reproductive status is heralded by certain characters in their hard parts, the ampullae, which are exhibited only when they are reproductively active: the ampulla is a protective cove in the skeleton that houses the medusae, the reproductive individual that carries forth the gametes.  I reasoned that information regarding seasonality of reproduction could be learned by studying time series of their the hard parts.  

I have to admit that I have learned little about the hard parts of Millepora.  Much more could be learned.  

 

Recently I have been awakened to a seriously interesting aspect of the biology of Millepora spp.: the degradation of the tissues, and the zooxanthellae, especially in the lower layer of the living tissue.   This, in turn, has turned my attention to another aspect of the morphology of their skeleta:  as the tissues lay down more and more layers of the Calcium Carbonate---the skeletal tissue---the polyps produce a sequence of new basement plates.  Akin to the sequence of corallites in scleractinian corals, these plates leave behind a sequence of consecutive chambers, evidence of the progressive accretion of the massive skeletal structure, the corallum. 

These plates are termed the tabulae.  Interestingly, tabulae are also described for the extinct rugose corals, in the _Treatise on Invertebrate Paleontology_.  

 

I note, in part:

• The distance between successive tabulae varies.  One wonders to what extent growth rate of the corallum is determined by, or alternatively determines, the gaps between deposition of these tabulae.  
• The surface of the colony is planar.
• Structure is organized at several levels: (1) gross morphology (tabulae and apparent palisades forming a corallite wall-figure 5);  (2) orthogonal crystalline (trabulae?)--figure 3; and over the broken surface may be seen grains, as in figure 1 and others.
• thick bands orthogonal to the tabulae are visible 

 

Photographs

The following photographs are presented.  It is planned to continue to capture other views of the hard parts of Millepora spp.

 

1

2

3

4

5

Much remains to be learned from studies of the skeletal remains of Millepora platyphylla.

These images were made with a Canon EOS M50, with the 28mm EF-M macro lens.  This is the maximum magnification with this lens, without additional add-on supplementary lenses.  This lens does not accept an extension tube.

 

 

Note to me: Processes during development of the Ova in the medusa

My interest in Millepora spp. (or just Millepora) begins with reproductive timing.  My interests range into seasonality and annual cycles, lunar cycles, daily (circadean) cycles.  I started paying attention to Millepora when I learned that the Ampullae were present in the hard parts (the rock, the skeleton) during reproduction; thus, if I collected a few pieces of their skeleta every time I went to the reef, I would eventually have a record of reproductive activity, enabling deeper understanding into the seasonality of this hydrozoan coral.  I was actually interested in neurosecretory control of gametogenesis,and had hoped to study giant clams.  But these charismatic and delicious macro-molluscs were too few and too far between for such a study on Guam.   Millepora was an interesting option.  Although I did not intend this to be my thesis topic, thse proved to be an intensely interesting organisms.

One night, I was invited to tag along on a night fishing foray with  friends from Pingelap Island.  That night, in early April, 1985, I was thrilled to observe Millepora shedding medusae!   The next day, I drove around the Western side of Guam and sampled Millepora at several sites: these sites also proved to be reproductively active. 

So, it was actually possible to observe reproduction in real time.  My study caught fire at this point.  I started collecting tissue specimens along with the pieces of skeletal material.  I made a point of collecting paired specimens from each colony, and I marked the colonies. 

I have some 200 specimens, collected from different colonies at different times over two seasons on Guam.  Much of my second year at the University of Guam was spent working on these specimens.  Over 200 tissue samples were fixed and embedded in paraffin.   John Starmer rescued them from Guam, when they were about to be thrown out.  

To make a long story short, my interest in Millepora has been focused at the microscopic level.  While I've been trying to grok tha variables involved in attaching a camera to a microscope, I have been focusing on primarily events at the level of developing medusae.  Millepora  hydrocorals are hydroids, that produce ephemeral medusoids (short lived free swimming medusae that exist to carry gametes off and spawn).  

Is it not obvious that the timing of the development of these medusoids is part of the complicated dance that led to the synchronized liberation of medusae observed on the reef that night, at Toguan Bay, Guam.

Here I will post some photos made with a cell phone of a slide from a specimen collected on 9 April 1986, the night of the New Moon.  I have made some assumptions about the timing of reproductive events in Millepora.

 Assumptions (for Millepora platyphylla)

1. Liberation of Medusae is sychronous, on the same night.  At several sites on Guam, liberation was evident on the same night, as far as I could tell.
   
2. Medusae are released/liberated (or alternately break free) on the fourth or fifth evening after Full Moon.
3. Reproduction is seasonal.  The season is about three or four months long.  The first event is in early April or late March.
   
4. Reproduction happens each month during season.  

 

Millepora dichtoma was reproductively active later in the Summer.