Study of Blastula Stages in Mammals


To study the blastula stage of embryonic development in mammals, with the help of permanent slide. 



The blastula stage is a crucial phase in embryonic development, characterized by the formation of a hollow sphere of cells. In mammals, this process involves several key steps.   


  • The blastula stage begins with the fertilization of an egg (ovum) by a sperm cell. 
  • This forms a zygote, which is the initial diploid cell containing genetic material from both parents. 


  • The zygote undergoes a series of rapid cell divisions known as cleavage. 
  • These divisions produce a cluster of cells called a morula. 

Morula Formation: 

  • As cleavage continues, the morula transforms into a solid ball of cells.

Blastocyst Formation: 

  • Further cell divisions lead to the development of a blastocyst. 
  • The blastocyst consists of an outer layer of cells (trophectoderm) and an inner cell mass. 

Hollowing and Blastocoel Formation: 

  • The blastocyst undergoes a process called cavitation, where a fluid-filled cavity called the blastocoel forms within the inner cell mass. 
  • This results in the transformation of the solid mass into a hollow sphere. 

Blastocyst Differentiation: 

The inner cell mass differentiates into two main cell layers 

  • The embryoblast (or inner cell mass) gives rise to the embryo. 
  • The trophectoderm contributes to the formation of extraembryonic tissues, including the placenta. 


  • The blastocyst undergoes implantation into the uterine wall. 
  • This process is essential for the establishment of a pregnancy. 

Gastrulation (Subsequent Stage): 

  • Following implantation, gastrulation occurs, leading to the formation of three germ layers: ectoderm, mesoderm, and endoderm. 
  • These germ layers give rise to various tissues and organs in the developing embryo. 

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. 

It's important to note that the blastula stage is a transient phase in embryonic development, and it marks the transition from the early cleavage stages to the more complex processes of gastrulation and organogenesis. The specifics of these stages can vary among mammalian species. 


Learning Outcomes 

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