why is the cleavage in frogs holoblastic and not meroblastic?

Question

background information
The cleavage of the frog embryo during the embryo development is holoblastic (complete cleavage). However when we look at yolk-rich eggs we see a cleavage pattern which is meroblastic (partial cleavage).

Question
But the frog eggs are also yolk rich right? So why do frog embryo's use holoblastic cleavage?

Answer 1

Because:

1. Frog eggs are not yolk-rich. They are mesolecithal, they have an intermediate amount of yolk.
2. Yolk is concentrated within vegetal pole of the egg. That is why yolk-packed macromeres divide much slower than micromeres on the vegetal pole, which barely contain any yolk.

My sources:

• https://kjcscientific.wordpress.com/project-excalibur/
• Gilbert's Developmental Biology (2009)