Marsupial oocyte classification

Egg classification is based on two criteria:

1. Amount of yolk
2. Distribution of yolk

Alecithal is a type of egg under classification based on amount of yolk while telolecithal falls under the classification that is based on the distribution of yolk.

Alecithal egg has no or little yolk while telolecithal egg has yolk concentrated at either of the two poles.


Most of the placental mammals have alecithal eggs. The cleavage is holoblastic. But, that's not always the case with non-placental mammals (that includes marsupials). Though I haven't looked your link up, but if they are saying that marsupial eggs are telolecithal, I won't disagree. But, if you are asked that on an exam, write alecithal and add moderately telocithal (at you own risk! )

P.S. If you write telolecithal only, the examiner might also mark it incorrect because telolecithal eggs undergo meroblastic cleavage. Mammals undergo holoblastic cleavage.

I'll check this up with my zoology prof.

I don't think there are any sources which argue the marsupial oocytes are straight telolecithal. Clearly it's not due to its holoblastic cleavage, as you pointed out. The article I linked seems to classify the marsupial as "moderately" telolecithal, meaning a shade inbetween telolecithal and alecithal. In so-called moderately telolecithal eggs, there is a significant deposit of yolk, but not enough to make cleavage meroblastic.
So my question remains as to whether the marsupial egg is alecithal or moderately telolecithal, but I agree that telolecithal is certainly not the case.
Maybe it simply depends on the specific marsupial species.

I don't know if this is going to be of any help, but I thought I would just chime in.

Are you aware of the mammalian classification? Marsupials belong to the subclass Theria. Further under Theria, there's an infraclass Metatheria and
marsupials, to be exact, belong to Order 2 Marsupilia. This is the most primitive order under the concerned infraclass in that its members lack placenta while from Order 3 on (which is Insectivora) the creatures are all placental.

The interesting part is that Order 1(Monotremata), which is the sole order of subclass Prototheria, precedes Order 2 (Marsupilia) in terms of advancement and monotremes have telocithal eggs. Catch on -- animals of Order 3 (Insectivora), which is next to Marsupilia, produce alecithal eggs.

I hope you get the point. There is a good possibility that the marsupial oocyte is somewhere between the two.

I will surely check this up with my prof. tomorrow and come back here if I am able to glean something significant.

P.S. The class Monotremata includes mammals of the like of Echidna, Platypus etc -- all peculiar to the Australian continent.

Patagonian Platypus (extinct I believe). Long beaked Echidna, New Guinea.



Are there other monotremes?


But thats all off the topic sorry spidy and levi.

egging you on.

Echidna is native to New Guinea and Australia.

http://en.wikipedia.org/wiki/Echidna



New Guinea is not politically a part of the Australian continent but geographically quite proximal to it. The point was that monotremes are restricted to that particular region of the earth.


As for platypus, I was referring to Duck-bill Platypus (binomial name : Ornithorynchus anatinus.)


Other monotrmes? None that I know of. Echidna and Platypus were the only specimens shown to us during class. I added etc. at the end because I wasn't sure myself.

Okay, my prof. said it's alecithal. Most references say so.