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.
- 11 Jun 2017
Formation of endoderm: Endoderm of hypoblast develops as a single layer of cells in side of blastocoel. After the formation of endoderm, upper layer is called epiblast. There are different theories to explain the formation of endoderm.
Infiltration theory: This was proposed by Peter in 1923. According to this theory some cells in blastoderm which are loaded with yolk will fall into blastocoel. It starts from posterior end of blastoderm. From there the cells migrate forward one behind another and endoderm is formed.
Delamination theory : It was proposed by Spratt in 1946. Blastodenr. is two or three layered thick. The lower layer will separate-fcoijl the upper layer by splitting and the lower layer is called endoderm, upperr layers are called ectoderm. In between ectoderm and endoderm blastocoel is present.
Theory of involution: In 1909 Peterson Proposed this theory. According to this theory a slit like opening at the posterior side of blastoderm forms. Through this opening the blastoderm cells will role into the primary blastocoel. It forms an endoderm.
Theory of invaginaton: This was proposed by Jockobson in 1938. According to this theory the posterior end of blastoderm will invaginate in blastocoel as a small pocket. This becomes endoderm. In this way endoderm is formed.
Laying of the egg: Between 9 A.M. and 3 P.M., the egg is expelled from the cloaca of hen. At the time of laying formation of endoderm is completed. For further development it is to be incubated.
Incubation: When the egg is laid, the development is stopped. For further development it is to be kept at 38°C. This is done by hen by sitting over the egg. This is called incubation. Artificially eggs are incubated in incubators. For the hatching of egg 21 days are required.
In the upper region of oviduct fertilization will takes place. One sperm will penetrate into hens egg and fertilizes with the egg. The fertilized egg will travel through oviduct. It takes nearly 22 hours. Hence the early development of egg will take place in oviduct.
Cleavage: Cleavage is restricted to blastodisc. The presence of great quantity of yolk the cleavage is restricted to blastodisc. Such cleavage is called meroblastic or discoidal cleavage.
1. Cleavage: After five hours of fertilization the first cleavage will appear. It is confined to the centre of blastodisc. Itismeridional.lt cannot completely divide the blastodisc. Blastomeres are not formed.
2. Cleavage: It takes place at right angles to first cleavage. Even because of second cleavage clear blastomeres are not formed.
3. Cleavage: It is vertical. It is in the two sides of first division. As a result of this division eight blastomeres are formed. But they do not show boundaries.
4. Cleavage: It takes place in such a way that eight central blastomeres and eight peripheral blastomeres will form. Only at this stage of division definite cells are formed. The central eight cells are completely separated from yolk.
After fourth cleavage the cleavages are irregular and a blastoderm is formed.
As the central cells are forming below these cells a horizontal cleft is formed. Because of this an upper layer of cells and a segmentation cavity is formed. This segmentation cavity is called blastocoel. These cells will undergo further division quickly. Hence above the segmentation cavity mass of cells will be present, in several layers. These cells have complete boundaries. The cells present towards the periphery are not separated from yolk. They are called marginal cells. This region is called zone of junction.
Area Pellucida & Area opaca:
The central cell mass of the blastoderm will be in four to five layers, they are lifted from the yolk. Hence the central part of the blastoderm is free from yolk. This region is transparent. It is called area pellicida. At the zone of junction the cells are in contact with that region is opaca. That region is called area opaca.