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Egg of toad
Cleavage
Axis of cleavage
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.
Female genital System
1. Cleavage:
1st cleavage: Vertical and give two blastomeres.
2nd cleavage: At right angle to the first and give 4
blastomeres.
2rd cleavage :Is radial at right angle to the second.
4th 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.
nd
2 : Occurs at prependicular axis at 60 hours and result in the formation of 4
blastomeres.
3rd : 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 12th 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 6th 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.
Embryonic Membranes
Found only in amniotic animals and formed
mainly of 4 types;
1. A mnion.
2. Chorion.
3. Allantois.
4. Yolk Sac.