The first line of defence (or outside defence system) includes physical and chemical barriers that are always ready and prepared to defend the body from infection. These include your skin, tears, mucus, cilia, stomach acid, urine flow, 'friendly' bacteria and white blood cells called neutrophils.
They are the managed consumption of natural resources to prevent their depletion or the destruction of the environment.
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Answer:
C - Contains the embryonic disk
Explanation:
After an egg is fertilized, it begins to divide into smaller cells, from the 1 cell stage, to 2, to 4 and so on. Then, on the third day, the cells fall within 16-200 cells and it is then called a <em>morula</em>.
On the fifth day, the morula becomes a ball of calls which is now differentiated into an <em>inner cell mass and a trophectoderm</em>. The inner cell mass becomes the <em>embryo</em> as development goes on and the trophectoderm, becomes the <em>placenta</em>.
As development goes on further, the floor of the amniotic cavity is formed by the embryonic disk, a layer of prismatic cells derived from the inner cell mass.
The embryonic disc develops into 3 layers:
There is a major differentiation of cells which gives a template for the origin of development of specific organs of the embryo.
