Short / Long Answer question Type :
Describe the development of fertilized ovum in humans up to the formation of germ layers.

• Two celled embryo/zygote undergo a series of divisions.
  
• One cell divides first so that a 3 cell stage of the embryo is formed, followed by a 4-cell stage, a 5-stage cell etc. This process of subdivision of the ovum into smaller cells is known is cleavage.
  
• As cleavage precedes the ovum have 16 cells. It is called as the morula. It is still surrounded by the zona pellucida.
  
• In a cut section of the morula, an inner cell mass is seen which is completely surrounded by an outer layer of cells. The cells of the outer layer give rise to a structure termed as the trophoblast.
  
• The cells of the trophoblast help in providing nutrition to the embryo.
  

• Some fluids passes into the morula from the uterine cavity and partially separates the cells of the inner cells mass from the trophoblast.
  
• As the quantity of fluid increases, the morula acquires the shape of a cyst.
  
• The cells of the trophoblast become flattened and the inner call mass get attached to the inner side of the trophoblast on the one side, Now the morula is termed as blastocyst.
  
• The trophoblast has the property of being able to stick to the uterine epithelium or endometrium and trophoblast cells have the capacity to eat up other cells. So they invade and burrow into the tissues to which they have come in contact.
  
• As the embryo travels down the uterine tube and the uppermost part of the uterine cavity, it is prevented from sticking to the epithelium by the zona pellucida.
  
• During this period embryo receives nutrition partly from the substances stored within the ovum and partly by diffusion from the uterine secretions.
  
• By the time a blastocyst is formed embryo needs additional sources of nutrition.
  
• This nutrition is received, when the blastocyst sticks to the uterine endometrium and gets implanted in it.
• Zona pellucida disappears soon after blastocyst reaches the uterine lumen.
• Function of the zona pellucida is to prevent implantation of the blastocyst at an abnormal site.
• As the blastocyst develops further it gives rise not only ot tissues and organs of the embryo but also to a number of structures that supports the embryo and help it to acquire the from of a 3 layered disc. This disc is called Embryonic disc.
• The 3 layers that constitute this embryonic disc are :

1. Endoderm

2. Ectoderm

3. Mesoderm.