Final answer:
The statement about the avian blastula consisting of two layers of cells is incorrect; in avian development, it refers to a blastodisc with a single layer. In mammals, the blastula transitions to a blastocyst, which has two distinct cell layers: the inner cell mass and the trophoblast.
Step-by-step explanation:
The avian blastula is a stage in embryonic development consisting of a single layer of cells surrounding a fluid-filled cavity known as the blastocoel. However, there appears to be a mix-up in the question as it references the development of mammals as well. In mammals, the blastula progresses to form the blastocyst, which has two distinct cell layers: the inner cell mass, which eventually develops into the embryo, and the trophoblast, the outer layer that contributes to the formation of the placenta.
The inner cell mass consists of embryonic stem cells which will further differentiate into various cell types required for the organism's development. In contrast, the trophoblast is involved in establishing the nutrients' exchange with the mother. Rapid cell division without overall mass increase, known as cleavage, leads to the formation of the blastula from a single-celled zygote. It's important to clarify that in the context of avian development, the term should properly refer to a blastodisc rather than a blastula.