The second step in gastrulation is the formation of primitive streak. At the posterior region of area pellucida in the mid dorsal line primitive streak will appear as a thickened area. It starts eight hours after incubation. The thickening is because convergence of cells of blastoderm towards the centre. Usually in the early stages the primitive streak is short and broad. It is called primitive streak. It gradually extends to the middle of blastoderm. At eighteen to nineteen hours of incubation, primitive streak is well developed. It is caikd definite primitive streak. Along the middle line of primitive streak a narrow furrow is develop called Primitive groove. The edges of groove are thick. They are called primitive folds. At the anterior end of groove a mass of closely packed cells will be present. It is called "hensen's node" or primitive knob. In the centre of this node a pit is present. It is called primitive pit. It represents the vestige of neurenteric canal. The primitive streak elongates along with this, area pellucida will also elongates. As the primitive streaks growing the cells from this region will invigilate into space between epiblast and hypoblast This process is called immigration. The immigrated cells will become prechordal plate, notochord, and mesoderm. Anterior to primitive streak the mesoderm cells will not migrate to primitive streak. This mesoderm free area is called proamnion. At this place head will develop.
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Shape: The 48 hours chick embryo will show this shape? (In mirror view). In this stage of development the head region shows prolific growth. Because of which the embryo shows this peculiar shape.
Flexure: In the head region bendings are seen. These bandings are called flexure. This bending is towards right and backwards. In 48 hrs. Chick embryo two flexures are seen. One is at the region of midbrain. Because of this fore and hind brain will become parallel. This is called cranial flexure.
In the posterior region of brain a cervical flexure is formed. This flexure turn the head towards right. This region will give neck of adult.
Torsion: The 24 hours chick embryo is flat. Its ventral surface is in contact with yolk. As the development proceeds the anterior end of embryo is turns towards right and hence the anterior part of embryo comes to lie on the left side of yolk. This twisting is called torsion. At the end of 48 hours chick twisting will reach the cervical flexure. It is nearly at 13th somite.
Brain: The brain, shows three regions. Fore brain shows telencephalon and diencephalon, the hind brain shows medulla oblongata.
Eye: The optic cup is two layered. Lens placode is started its development.
Ear: The invagination of optic vesicle is completed. It is connected to ectoderm by endolymphatic duct.
Alimentary canal: Fore gut reaches 1/2 mm length.
Heart: It is 5' shaped. It is tubular. !t has not developed chambers sets.
Blood vessels: From the anterior end of heart two ventral aortae will arise. They run backwards and travel as dorsal aortae.
Kidney: Pronephrous is completely developed. Mesonephric tubules started development.
Somites: At 48 hours of incubations 25 pairs are seen. Mesonephric tubules will develop from intermediate mesoderm.
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At 24 hrs. incubation period the chick embryo is oval in shape. The primitive streak is seen. From the hensen's node a notochord is developed.
In the 24 hrs. chick embryo four pairs of somities are present.
NEURALFOLDS: On either side of mid dorsal line the ectoderm becomes thick. It forms longitudinal folds. They are called neural folds. They include a space called neural groove, this neural groove is deep in anterior region, and the depth decreases gradually posterior region. Near the first somite region the folds will startrfusing. It will start after 24 hours of incubation. Posteriorly diverse. This area is called sinus rhomboidalis. In this region regressing primitive streak is developed.
GUT: At this incubation stage foregut is formed. Mid and hind guts are not formed. The opening of foregut into midgut is called anterior intestinal portal on either side of this the splanchanic mesoderm becomes thick.
MESODERM: It is present between ectoderm and endoderm. It occupies the entire blastoderm.
BLOOD ISLANDS: Around the area pellucida, area opaca is present. In this opaca region groups of mesenchyme cells will present. They are called blood islands. They produce embryonic blood vessels.
AREA VASCULOSA & AREA VITELLINA: In the area opaca the region which contain blood islands, is called area vasculosa. The peripheral area without blood vessels is called area vitellina.
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EMBRYOLOGY OF CHICK
The following list clearly describes about embryological changes in chick egg during incubation.
- FRESHLY LAID HEN'S EGG
- CHICK EMBRYO AT 24 HOURS
- CHICK EMBRYO AT 48 HOURS
- GASTRULATION IN CHICK-I
- GASTRULATION IN CHICK-II - FORMATION OF ENDODERM
- GASTRULATION IN CHICK-III - FORMATION OF PRIMITIVE STREAK & MESODERM
- GASTRULATION IN CHICK-IV - DEVELOPMENT OF MESODERM AND COELOME
- CHICK: EXTRA EMBRYONIC MEMBRANES