Gastrulation Gastrulation carried out byMigration of cells from the surface Toward the blastopore (epiboly), followed By invagination and involution. The result is the formation of three germ layers: 1.Outre ectoderm. 2.Middle mesoderm. 3.Inner endoderm.
Embryonic Development of Birds Fertilization : Internal. Type of egg : According amount of yolk : Polylecithal type. According to distribution of yolk: heavy telolecithal type. Kind of cleavage : Discoidal cleavage.
1.Cleavage: 1 st cleavage: Vertical and give two blastomeres. 2 nd cleavage: At right angle to the first and give 4 blastomeres. 2rd cleavage :Is radial at right angle to the second. 4 th cleavage: Central arround the apical end of the eight blastomeres. The cleavage proceeds with irregular manners with increased in the number of cells and spread over the surface of the yolk.
Formation of Blastula: The blastomeres spread over the yolk surface with closely attachement to the yolk by both ends and the zone of attachement called zone of junction. The blastocoele is reduced and is called subgerminal space or segmentation cavity. Formation of Gastrula: The blastoderm cells arranged in different cell layers. The innermost cell layers separated by a process called delamination forming outer epiblast and inner hypoblast which is known afterwords the ectoderm and endoderm. Or by different process of cell movements called: Emboly: convergence, invagination & involution. Epiboly: include overgrowth of the ectoderm and endoderm over the yolk mass.
Embryonic Development of human Fertilization: Internal. Type of eggs : Alecithal type. Type of cleavage: Equal holoblastic. Cleavage: Ist: occurs at right angles to imaginary axis and results in the formation of two blastomeres after 36-46 hours. The embryo still surrounds by zona pellucida to make close contact of the blastomeres. 2 nd : Occurs at prependicular axis at 60 hours and result in the formation of 4 blastomeres. 3 rd : Occurs at 70 hours and result in the formation of 8 blastomeres. At 72 hours : The morula stage is formed and composed of 12 cell stage.The peripheral cells is called trophoblast and formed of 11 cells and the 12 th cell is very large and enclosed in the center and called inner cell mass. At 4 days : 58 cell embryo is formed. The outer periphery trophoblast cells is formed of 53 blastomeres. The inner cell mass composed of 5 large cells. At 4.5-5 days: 107 cell stage is formed with peripheral trophoblast cells composed of 99 cells and inner cell mass composed of 8 formative cells (inner cell mass).
Implantation: At the 6 th day, the zona pellucida become lysed after the blastula reached to the proper side of the endometrium. The trophoblast cells exert microvilli which intern exert a histolytic action on the endometrium epithelium facilitating penetration and implantation. By the end of 8 days implantation is completed. After implantation the eendometrium is closed by fibrin clott. Implantation occurred at the posterior wall of the uterine tube near the fundus. Abnormal site of implantation (Ectopic pregnancy): Tubal pregnancy. Ovarian pregnancy. Abdominal pregnancy.
Gastrulation: The inner cell mass divides into several cell layers. The outer is the ectoderm. The middle is mesoderm. The inner is the endoderm. Periods of pregnancy: First trimester: First 3 months. Second trimester: Middle 3cmonths. Third trimester : The last 3 months.