Some of the major steps which are involves in the development of chicks are as follows:-
- Fertilization :
Ova leave the ovary called ovulation as primary ooctyes. They are released in coelom and caught by the expanded funnel, like opening of oviduct. They are fertilized in the upper part of the oviduct which is also receives sperms from the male bird during copulation, thus the fertilization in birds is internal. Ovum or egg cell contains very large amount of yolk and the egg is an example of polylecithal, macrolecithal and telolecithal type.
The cytoplasmic materials is very little and is in the form of a small disc over the yolk called blastodisc or germinaldisc. The nucleus of only one sperm fuses with the nucleus of ovum properly to achieve the fertilization. The mere entry of sperm in the egg membrane induces the oocyte to undergo two quick maturation divisions. Cleavage starts soon after and the egg begins to receive tertiary membranes.
- Cleavage :
Cleavage begins about 3 hours of fertilization. Cleavage and early gastrulation completes by the time the egg is laid.
Because of the enormous yolk not one cleavage furrow is able to penetrate through the yolk and divide the egg completely, hence the cleavage divisions are limited to the small blastodisc, and such a type of cleavage is known as partial, teloblastic or meroblastic.
First cleavage division is vertical and divides the germinal disc through superficially only, second division soon follows this too i.e. vertical but at right angles to the first. These several similar subsequent division do not extend right across the germinal disc; thus, the early cleavage is confined to a central part and soon the germinal disc comes to have a central mass of cells and a peripheral undivided cytoplasm, the periblast or marginal zone.
Later division extends into the periblast and cut-offs larger and less numerous marginal cells, which not only add to the cells, but also extends directly over the yolk. Divided blastodisc is hereafter is called blastoderm.
- Blastulation :
The free margin of blastoderm grows rapidly over the surface of yolk. A small fluid filled space appears just beneath the central mass of cells, this is the sub-germinal cavity,often called blastocoels though not real. The germinal disc when seen entirely it appears to have a transluscent central area, the area pellucid, because of the fluid present inside the subgerminal cavity beneath it and an area opaca around the former, because of the solid yolk underneath it.
Area pellucida later forms the body of the embryo proper, while the area opaca forms the accessory extra embryonic structure, such as the yolk sac. The subgerminal cavity is not being a true blastocoels, this stage in chick is called a pseudoblastula.
Before gastrulation the multilayered blastoderm of area pellucid becomes rearranged to form a single layer of cubical epithelial cells called epiblast or ectomesoderm. On this layer the presumptive areas have been mapped which later form the main embryonic structures.
Following are the presumptive areas, starting at the future posterior end of the embryo;
- a small disc of endoderm.
- a broad rear band of lateral mesoderm.
- a narrow band of notochordal mesoderm.
- a small area of prechordal mesoderm flanked on either side by somatic mesoderm.
- a large crecentric area of neural ectoderm.
- a large area of epidermal ectoderm. All these presumptive areas lie within the area of pellucid. Embryonic ectoderm is continuous with extra embryonic ectoderm on area opaca, which covers extra embryonic endoderm.
- Gastrulation :
Gastrulation begins even before the laying of egg. It involves the formation of endoderm so that the monoblastic embryo or blastula get converted into diploblastic oe two-layered gastrula.
There is no invagination of prospective endoderm through a blastopore as found in frog. Prospective endoderm cells from posterior end of the embryo, migrate down ( involution ) into the subgerminal cavity forming a coherent sheet throughout the area pellucid.
There is also to some extent, a sinking down or ingression of indivisual yolk laden ectodermal cell thus adding to the thickness of blastoderm. The later now splits away or delaminates from the inner side, the process being known as delamination. Delaminated cells destined to form embryonic endoderm and are reffered to as hypoblast and the outer as epiblast (ectoderm).
The epiblast contains the prospective neural plate, notochord, mesoderm and ectoderm. The embryonic endoderm formed by migration of cells from area pellucid into subgerminal cavity, lines the gut of embryo, whereas yolky or extra-embryonal endoderm formed by area opaca lines the yolk sac. Gastrulation is completed within 2 to 3 hours after laying. Withi its completion, the embryo becomes diploblastic containing an outer epiblast (ectoderm) and inner hypoblast (endoderm).The original subgerminal cavity is divided by hypoblast into a narrow outer space or secondary blastocoels and inner archenteron or primitive gut of embryo.
I ) Cleavage :
After 2 to 3 hours of fertilization, the zygote begins to divide. The repeated division in successive fashion is known as cleavage or segmentation.
Division is mitotic, the cleavage begins as a small depression at animal pole and gradually extends at surrounding of zygote, dividing into two cell.
The division are holoblastic and complete.
First cleavage is vertical; and it is two celled stage. Second cleavage is also vertical but right angels to the first one; forms 4 celled stage. The cells are known as blastomere. Third cleavage is horizontal but above the equatorial line forming cells of unequal size. The upper 4 cells towards animal pole are small and pigmented known as micromere or epiblast. The lower 4 large yolk laden cells are known as megameres or hypoblast. Fourth and fifth cleavage are also vertical and forming 16 celled zygote. These division is followed by two horizontal cleavage, one towards vegetal pole, resulting in 32 celled stage.
II) Morula :
As a result of repeated and irregular cleavage, balls of cell is formed known as morula stage. One hemisphere of morula is composed of large number of small black and yolkless cell known as micromeres and other hemisphere is composed of fewer number of large white and yolk laden cells known as megameres.
III) Blastula :
The micromeres divides more rapidly than megameres which results in formation of small fluid filled cavity known as Blastocoel or segmentation cavity. Blastocoel bearing stage is called Blastula. The floor of the blastocoels is composed of layer of yolk laden megameres while the roof is composed of micromeres. In this stage early presumptive areas can be differentiated by staining technique. The entire animal pole of blastula represents the presumptive ectoderm which is further divided into presumptive epidermis and presumptive neural plate.
A small area near vegetal pole is presumptive notochord, close to presumptive there is a grey crescent region which is presumptive mesoderm. The remaining vegetal region is presumptive endoderm.
IV) Gastrulation :
Gastrulation is the two layered embryo stage formed by migration and rearrangement of cells of blastula. The process of formation of gastrula is called gastrulation.
Gastrulation involves some critical changes in blastula such as differentiation of cell, transformation from monoblastic to diploblastic layer, formation of three primary germ layers.
Gastrulation completes in following steps :-
- Epiboly :
In this step, micromeres at animal pole divides more rapidly enclosing the megameres except in the region of yolk plug. This overgrowth or spreading of micromere cells is known as Epiboly.
- Emboly : (Invagination) :
In this step, small groove appears due to invagination of megameres near grey crescent region. The invagination gradually grows inward causing migration of cells. This stage is also known as yolk plug stage.
The narrowing of blastomere excerts pressure on underlying yolk laden megameres, result in protruding of some megamere cells as yolk plug.
Contraction of lips of blasto pore, contraction of lips from all sides occurs so that blastopore become smaller and narrower. As invagination progresses archenteron increases in size and the blastocoel get reduced and finally obliterated. This groove is the beginning of archenteron and its interior opening is called blastopore. The blastopore is guided by anterior margin called dorsal lip and backward projecting lateral lip.