Phases of Development

Phases of Development
The hen’s egg undergo following phases during development
1. Cleavage
2. Morula
3. Blastula
4. Gastrula
5. Notochord Formation
6. Neuralation
1. Cleavage
The device of repeated mitolic divisions that takes place in the fertilized ovum is called Cleavage.
Types of Cleavage
There are two types of Cleavage
i. Discoidal Cleavage
ii. Meroblastic Cleavage
i. Discoidal Cleavage
The process of cell division is continued to the small disc of protoplasm lying on the surface of the yolk at the animal pole. This type of cleavage is called as Discoidal Cleavage.

• It occurs in birds

ii. Meroblastic Cleavage
The type of cell cleavage in which blastodisc divide completely is called Meroblastic Cleavage or incomplete cleavage.

• It occurs in Fishes and reptiles.
• The first two cleavage are vertical and the third runs horizontal parallel to the surface as a result 8 blastomeres are formed.
• The successive cleavages become irregular and number of cells increase. This complete body is called Blastoderm.

2. Morula
Defintion
Cleavage results in the formation of a rounded closely packed mass of blastomeres called Morula.

• It is without cavity.
• It consists compactly arranged cells called blastomeres.
• Morula is short stage, very soon it is converted into blastula stage.

3. Blastula
Definition
The embryonic stage which contains a fluid filled cavity, the segmentation cavity or blastocoel is called Blastula.

• By the appearance of cavity the cells of bolstoderm divide into two layers.

i. Epiblast
ii. Hypoblast
i. Epiblast

• The upper layer is called Epiblast.

ii. Hypoblast

• The lower layer is called Hypoblast.
• Between these two layers blastocoel is present.

a. Area Opaca
The marginal cells of blastoderm lie unseparated from the yolk and form the zone of function called Area Opaca.
b. Area Pellucida
The middle or central region of balstoderm is called Area Pellucida.

• Egg is laid almost at this stage.

4. Gastrula
Definition
The embryonic stage in which movement and rearrangement of cells at definite location in the embryo occurs is called Gasrula and the process of formation gastrula is called Gastrulation.
In this stage, cell begins to differentiate into definite layer which are
i. Endoderm Formation
ii. Mesoderm Formation
iii. Ectoderm Formation
i. Endoderm Formation

• Endoderm is formed from lower layer hypoblast of blastoderm.
• Cells of area of pellucida (hypoblast) rearrange and migrate to the lower region and spread over the yolk to form floor of sub-germinal cavity.
• This newly form layer called endoderm, which consists of yolk and stalk.
• Stalk connects yolk region to embryo.
• Invagination and archenteron are not formed in this type of gastrulation.

ii. Mesoderm Formation

• Mesoderm is middle layer and is produced from upper region of blastoderm called Epiblast.
• Invagination of Epiblast does not take place.
• The cells of epiblast move downward in the middle of blastodisc, the separates and move inward towards the yolk.
• The cells are migrated from both sides and form a thick central line called primitive streak.
• The upper end of primitive streak is swoolen called primitive knot or Hensen’s node.
• The cells of epiblast pass through primitive streak and move in blastocoel cavity form both sides and form a new layer called Mesoderm.
• Due to migration of mesodermal cells, from epiblast a groove is development on the entire upper region of embryo called primitive groove.

iii. Ectoderm Formation

• The remaining cells of epiblast after migration of mesoderm form the surface layer called Ectoderm.
• At the end of this stage, embryo consists of three germinal layers.

5. Notochord Formation
After the formation of primitive streak, the cells of Hensen’s node are migrated to the inner side and form a rod-like structure in the middle region it is notochord. It lies below the neural plate.

• Notochord is formed in emrbyo in 18 hours.
• It develops into vertebral column.

Differetiation of Mesoderm
After 24 hours of incubation, the lateral plate of mesoderm is differentiated into two types of layers

• Somatic layer => Outer layer
• Spianchnic layer => Inner layer

6. Neuralation
Formation of Neural Plate

• After gastrulation, the ectodermal cells of middle part of dorsal region divide rapidly to form a thick plate called a Neural Plate.

Formation of Neural Flods

• This plate moves downward and its edges arise on both sides to form two folds, known as Neural Folds.

Formation of Neural Tube

• These folds move towards each other, unite in the middle and produce a tube, known as Neural tube.

Neurula
At this stage the emrbyo become elongated and called neurula.

• The upper part of neural tube helps in the formation of brain.
• The remaining tube forms the spinal cord and its cavity called the neurocoel.