The Theory 

Cleavage and Blastula Stage 

The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage.   

The cleavage divisions do not involve an increase in the overall size of the embryo. Instead, the zygote undergoes multiple rounds of mitotic cell division without cell growth in between. With each division, the individual cells, called blastomeres, become progressively smaller due to the lack of cell growth. The result of cleavage is the formation of a solid ball of cells with no empty spaces or cavities. This stage is called a morula. 

The blastula is an early stage of embryonic development in animals. The blastula is formed through multiple rounds of cell division (cleavage) of the fertilized egg. During cleavage, the zygote undergoes rapid mitotic divisions without significant cell growth. As a result, a hollow ball of cells called the blastula is formed. 

The blastula typically consists of a single layer of cells, known as the blastoderm or blastodisc, surrounding a central fluid-filled cavity called the blastocoel. The cells on the outside of the blastula are referred to as the outer or surface cells, while those lining the blastocoel are called the inner or embryonic cells. 

The trophoblast is a layer of cells that forms the outer layer of the blastocyst, which is the early stage of development in mammals following fertilization and cleavage. The trophoblast is a significant structure in embryonic development, particularly in the context of implantation and the formation of the placenta. The trophoblast is derived from the outer cells of the blastocyst during embryonic development. These cells are distinct from the inner cell mass, which will go on to form the embryo proper. 

Gastrulation 

The blastula stage is crucial in embryonic development, as it marks the transition from a single-cell zygote to a multicellular embryo. It represents the early stage of development during which the embryo prepares for the next critical phase called gastrulation. The embryo undergoes gastrulation, a process in which cells within the blastula rearrange and differentiate into the three primary germ layers: ectoderm, mesoderm, and endoderm. Gastrulation sets the foundation for the development of various tissues and organs in the organism. 

Germ Layers 

Each of the primary germ layers formed during gastrulation gives rise to specific tissues and organ systems: 

•     Ectoderm: Gives rise to the nervous system, skin, hair, and other surface structures. 
•    Mesoderm: Forms structures like muscles, bones, the circulatory system, and reproductive organs. 
•    Endoderm: Gives rise to the lining of the digestive and respiratory tracts, as well as various internal organs. 

Learning Outcomes

• Students can understand the concept of the formation of blastula. 
• Students understand the steps involved in germ layers. 
• Students will learn the discrete stages of cleavage which leads to the formation of the blastula.