>
> I wrote a little about the topic of the marsupial placenta in my 1993 book,
> "Fauna of Kakadu and the Top End", a publication still used as a text by the
> University of NSW.
>
> Denise
>
> Denise Lawungkurr Goodfellow
> PO Box 71
> Darwin River, NT, Australia 0841
>
> PhD candidate, Southern Cross University, Lismore, NSW.
>
> Founding Member: Ecotourism Australia
> Founding Member: Australian Federation of Graduate Women Northern Territory
> 043 8650 835
>
>
>
>
>
>
>
>
>
> On 25 Sep 2014, at 7:39 pm, Stephen Ambrose <> wrote:
>
>> Marsupial embryos are attached to the wall of the uterus by a placenta in
>> the very early stages of development. The outer layer of cells in the
>> fertilised, dividing egg is called the trophoblast. When the egg implants
>> itself in the uterine wall, the trophoblast begins to differentiate into the
>> different tissue layers that form the placenta and invade the maternal
>> decidua (uterine lining). In eutherians (true placentals) the trophoblast
>> secretes chemicals (e.g. phosphocholine) that help the embryo avoid
>> detection by the maternal immune system, and develops suppressor cells which
>> inhibit the actions of maternal antigens that enter the body of the foetus.
>> Therefore, the foetus of eutherian mammals can remain implanted in the
>> uterus during its major phase of growth. On the other hand, the trophoblast
>> of metatherians (marsupials) has not evolved all the mechanisms to suppress
>> the actions of maternal antigens that occur on the surface of the uterine
>> lining. Therefore, although a marsupial placenta develops, there can only be
>> a brief period of attachment, otherwise the mother's immune system would
>> eventually kill the foetus. Hence, the need for marsupial embryos to leave
>> the uterus and develop as pouch young.
>>
>> The placenta in marsupials is derived from the part of the trophoblast that
>> forms the embryo's yolk sac. Hence, it is called the yolk sac placenta. In
>> eutherians, the placenta is derived from the allantois, which is another
>> embryonic sac that grows behind the yolk sac. Therefore, the eutherian
>> placenta is known as the allantoic placenta. The difference in the origins
>> of the eutherian and marsupial placentas probably explains why the latter
>> lacks some of the mechanisms to effectively combat the mother's immune
>> system.
>>
>> So strictly speaking, marsupials should be regarded as placental mammals and
>> I favour the use of the following clades (taxonomic groupings): Eutheria
>> (allantoic placental mammals), Metatheria (marsupials) and Prototheria
>> (monotremes).
>>
>> Stephen Ambrose
>> Ryde NSW
>>
>>
>> -----Original Message-----
>> From: Birding-Aus On Behalf Of
>> Jeremy O'Wheel
>> Sent: Thursday, 25 September 2014 2:42 PM
>> To: brian fleming
>> Cc:
>> Subject: Re: [Birding-Aus] FW: Bird tautology
>>
>> Marsupials aren't "placental mammals" either, although I think they do have
>> a placenta of sorts (but very small). Placental mammals are mammals in the
>> infra class "Placentalia".
>>
>> Jeremy
>>
>>
>>
>>
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