Q.Explain the process by which zygote divides and redivides to form the morula.

Step 1. The zygote, once activated after syngamy, begins cleavage — rapid mitotic division without any growth phase between divisions, so the blastomeres (cleavage cells) get progressively smaller with each division, and — because smaller cells have a higher metabolic rate — successive cleavages actually speed up.

Step 2. In humans, cleavage is holoblastic (the whole zygote divides each time) and radial/indeterminate (no blastomere's fate is fixed yet). The first cleavage, meridional (longitudinal), occurs about 30 hours after fertilization, splitting the zygote into two blastomeres of slightly unequal size.

Step 3. The second cleavage, also longitudinal but at a right angle to the first, occurs about 30 hours later, giving a 4-cell stage; the third cleavage is horizontal (equatorial), producing an 8-cell stage. …

• Assuming the embryo GROWS larger with each cleavage, when in fact overall size stays constant (only cell number increases) because there is no growth phase and the zona pellucida constrains it. …