2007-11-30 00:14:22 · 3 answers · asked by jasmine 1 in Science & Mathematics ➔ Zoology
In simple terms, it's because the amphibian yolk has to support the developeing fetus/embryo for all the time before it hatches. A mammalia embryo/fetus will get it's nourishment from the mother via a placenta before it is born. The obvious exception is the monetremes (egg-laying mammals) and a I suspect their ova have a lot more yolk.
2007-11-30 03:25:01 · answer #1 · answered by Anonymous · 1⤊ 0⤋
The answer to this question is quite interesting. Let's start with the definition of pregnancy: implantation of a fertilized egg on the uterine wall. This term is exclusive to placental groups. Amphibia is not a placental group, and therefore does not ever become pregnant, but rather the term for that group is "gravid." Because there is no placental activity, the nutrition obtained by the developing young must be entirely contained within the cell in which they develop. Mammalian young do develop with a placenta, and since they gather all of their nutrition from that, they have no need for a "yolk."
2007-11-30 03:20:32 · answer #2 · answered by crotalidman 2 · 1⤊ 0⤋
Amphibian (and bird for that matter) eggs need enough nourishment to provide for the growth of a baby. In mammals, the mother provides the nutrition needed until the baby is born (not hatched).
2007-11-30 00:19:31 · answer #3 · answered by Joan H 6 · 2⤊ 0⤋