Why Yolk is absent in human egg?

It's not entirely accurate to say that yolk is absent in human eggs. Human eggs do contain a tiny amount of yolk, but it's much smaller than the yolk found in bird or reptile eggs.

Here's why:

* Human embryos are nourished differently: Instead of relying on a large yolk sac for nourishment, human embryos receive nutrients directly from the mother through the placenta.

* Limited space: Human eggs are very small compared to those of other animals. There's simply not enough space within the egg for a large yolk sac.

So, while human eggs do have a small amount of yolk, it's not the primary source of nourishment for the developing embryo. The placenta plays that role, allowing the embryo to receive nutrients and oxygen directly from the mother's bloodstream.