Embryonic development upto three germ layers:

        Embryonic development includes a definite series of stages in which one celled zygote is transformed into multicellular and fully formed development stage till birth. It involves the following stages such as
a. Cleavage (formation of Morula)      

b. Blastulation  

c. Implantation 

d. Gastrulation 

c. Organogenesis

Cleavage: It is also called segmentation.

Definition. The process of rapid mitotic division of single celled zygote to form hollow, spherical, multicellular blastula is called cleavage.

Characteristics:

        i. Daughter cells are genetically similar to parental cell.

        ii. The resulting daughter cells formed are called as blastomeres.

        iii. Zona pellucida is intact, the size of morula is almost same as that of the fertilized ovum.

        iv. The size of blastomeres becomes smaller and smaller. So the cleavage is called fractionating process.

        v. As more and more blastomeres are formed the DNA content increases considerably with respect to the cytoplasm,

        vi. O2 consumption is also very rapid during cleavage.

Cleavage differs from mitosis in the respect that

        i. There is no growth phase between the successive divisions so cleavage decreases the cell size.

        ii. The metabolism becomes fast.

        iii. There is rapid DNA replication.

        iv. High consumption of Oxygen.

Types of Cleavage:

        There are two main types of cleavage.

        i. Holoblastic Cleavage. When whole egg is divided.

        It is found in microlecithal and alecithal egg.

        ii. Meroblastic Cleavage. When a part of egg is divided.

        It is found in polylecithal eggs.

        It may be discoidal (e.g. birds) or superficial (e.g. insects).

Cleavage:

      Period: It begins 3 hours after fertilization.

      Site: It occurs in fallopian tube when zygote travels towards uterus.

      Duration: It is completed in 72 hours after fertilization.

        In human, cleavage is complete or holoblastic and slightly unequal.

        It undergoes following cleavage,

1st Cleavage:

        It starts 3 hrs. after fertilization.

        It is completed about 30 hours after fertilization.

        It is meridional (vertical) i.e. it takes place from animal pole to vegetal pole.

        Two unequal daughter cells or blastomeres are formed

2nd Cleavage:

        It starts after 40 hrs. after fertilization.

        It is completed about 50 hours after fertilization.

        The second cleavage is also meridional (vertical) but at right angle to the first one.

        The second cleavage occurs first in one blastomere resulting in transitory three cells stage followed by four cell stage.

3rd Cleavage:

        It takes place two days after fertilization.

        It is longitudinal and horizontal (equatorial) and forms 8 cells.

        The cleavage continues and result in the formation of a solid mass of cells known as Morula.

Morula:

        It is about 16 to 32 celled stage, look like a small mulberry, so it is called morula.

        It reaches the uterus about 72 hours after fertilization (on fourth day).

        It is still surrounded by zona pellucida. It has two types of blastomeres.

1.    Micromeres: These are peripheral, smaller, and transparent

2.    Macromeres or Megameres: These are central and larger cells.

        It is nourished on uterine milk. Till implantation the developing embryo floats freely and receives nutrient secretion called uterine milk

        If the two first formed cells of the zygote separate they develop into identical twins

        If the two ova are fertilized by two sperms they form fraternal twins.

(REFER DIAGRAMS FROM THE TEXTBOOK)

VERY SHORT ANSWER QUESTIONS

1.      What is cleavage?

2.     What is morula?

3.     When cleavage takes place?

4.     What is the nature of cleavage in human beings?

5.     Which is the significant feature of cleavage division?

6.     Why cells become progressively smaller in cleavage?

7.     What are blastomeres?

8.     Why the size of morula is same as that of ovum?

9.     Name the layer of ovum which is retained even after fertilization.

10.  What is the nature of first cleavage?

11.   When the first cleavage is completed?

12.  What is the nature of second cleavage?

13.  When second cleavage is completed?

14.  What is the nature of third cleavage?

15.  Which structure is known as little mulberry?

16.  How many cells are present in Morula?

17.  What is the type of cleavage? / Why it is called intermediate cleavage?