Q18 - Reproduction in Marsupials

Question 1

What are the names of the four placentation types in metatherian mammals?

Answer 1

Types 1-4

Question 2

What is the general anatomy of the egg in type 1 placentation?

Answer 2

Embryo and small allantois are almost completely surrounded by yolk sac

Question 3

Which part of the egg is used for nutrient transfer in type 1 placentation?

Answer 3

Yolk sac (well vascularised)

Question 4

Why is type 1 placentation primitive compared to the other three types?

Answer 4

They use the allantois for nutrient transfer, type 1 uses primarily yolk sac

Question 5

How does the allantois differ between type 1 and type 4 placentation?

Answer 5

In type 4, the allantois is enlarged and fused with the chorion (chorio-allantoic placenta)

Question 6

How does type 4 placentation differ from the other 3 types to make it the most efficient in nutrient transfer?

Answer 6

It is the only type that involves invasion of maternal tissues, allowing for a greater degree of transfer

Question 7

What can provide evidence that type 4 placentation is more efficient in nutrient transfer than type 1?

Answer 7

Gestation time - shorter in type 4
Offspring size - larger in type 4
Lactation time - shorter in type 4

Offspring more able to fend for themselves and less of a burden on parents