Nutritional endoderm: a way to breach the holoblastic-meroblastic barrier in tetrapods.

The deceptively simple evolutionary transition from complete, holoblastic cleavage of the zygote to incomplete, meroblastic cleavage occurred only once in tetrapods, with the evolution of the amniote egg. By examining the development of a frog with large eggs, we identified a new tissue called the nutritional endoderm, which provides a possible intermediate step to breach the holoblastic-meroblastic barrier. Nutritional endoderm is divided into cells, but the cells disappear and do not contribute to tissues of the frog after the yolk is depleted. The complete loss of this yolk-rich tissue raises the question as to what feature of early development requires cellularization of this endoderm.