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| Original page: http://embryology.med.unsw.edu.au/Medicine/BGDlabfertilization11.htm | | Original page: http://embryology.med.unsw.edu.au/Medicine/BGDlabfertilization11.htm |
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| ==Introduction==
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| This page is a overview of events that occur in human development up to week 3 post-fertilization. From this Practical understand concepts of: fertilization, blastocyst development, implantation, bilaminar and trilaminar embryo formation, development of embryonic cavities and brief understanding of early placenta development.
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| ==Trilaminar Embryo==
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| By the end of week 3, segmentation of the 3 germ layers has begun:
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| # Ectoderm - central neural plate and lateral parts form epidermis
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| # Mesoderm - midline notochord, adjacent somites, formation of the internal embryonic space (intraembryonic ceolom)
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| # Endoderm - epidermal lining of gastrointestinal tract
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| == Carnegie Stage 8 ==
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| [[Image:Stage8 SEM1.jpg|240px|left]]
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| ===Facts===
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| Human embryonic stage 8 occurs during week 3 between 17 to 19 days.
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| The embryo is now 1.0 - 1.5 mm in size.
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| ===Events===
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| Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm.
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| Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.
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| From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube forms first the axial process then notochordal process, then finally the notochord.
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| The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation. It lies in the rostrocordal axis and the embryonic disc will fold either side ventrally, pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract.
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| ===Identify===
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| * embryonic disc
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| * primitive node, primative streak, primative groove
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| * connecting stalk
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| * cut amniotic membrane
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| [[Carnegie stage 8]]
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| == Carnegie Stage 9 ==
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| [[File:Stage9_sem1c.jpg|left]]
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| ===Facts===
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| Human embryonic stage 9 occurs during week 3 between 19 to 21 days.
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| The embryo is now 1.5 to 2.5 mm in size and somites have begun to form and number between 1 to 3 somite pairs during this stage.
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| The initial images are displayed unlabeled to allow you to explore the embryo for yourself, linked labeled versions are also available for some images.
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| ===Events===
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| '''Ectoderm''' - Neural plate brain region continues to expand, neural plate begins folding over the notochord. Gastrulation continues through the primitive streak region.
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| '''Mesoderm''' - Paraxial mesoderm segmentation into somites begins (1 - 3 somite pairs). Lateral plate mesoderm begins to vacuolate, dividing it into somatic and splanchnic mesoderm and to later form the intra-embryonic coelom. Prechordal splanchnic mesoderm begins to form the cardiogenic region, from which the primordial heart will develop.
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| '''Endoderm''' - Notochordal plate still visible which will form the notochord. Endoderm is still widely open to the yolk sac and germ cells form part of this layer. Extra-embryonic mesoderm on the yolk sac surface begins to form "blood islands".
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| ===Identify===
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| * Neural groove and neural folds, the mesoderm, which segments beside the neural groove to form somites but extends laterally to margin of embryonic disc lateral plate mesoderm, where it merges with the covering extraembryonic mesoderm.
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| * The intra-embryonic coelom develops in the middle of the lateral plate mesoderm. Note amniotic ectoderm covered by extra-emebryonic mesoderm (empty spaces above and below the mesoderm are artefacts, as are the lateral folds in the ectoderm).
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| The first two images using bright field microscopy approximate the orientation of the scanning electron micrographs below. There are additional scanning electron micrographs showing selected features in detail.
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| [[Carnegie stage 9]]
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| {{Template:Carnegie_stages}}
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| == Human Development Timeline ==
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| The table below shows human development features and approximate timing during the menstrual cycle to fertilization and the first 3 weeks of development.
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| The timing assumes fertilization the day after ovulation and the "weeks" refer to embryonic development and differ from clinical weeks (shown in brackets, from last menstrual period) and "stages" refer to Carnegie stages of development.
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| == Week -2 ==
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| (Clinical Week 1)
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| {| class="prettytable" width=100%
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| |-bgcolor="lightsteelblue"
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| | <center>'''Day'''</center>
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| | <center>'''Menstrual cycle'''</center>
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| | '''Event'''
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| | <center>1</center>
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| | Menstrual Phase
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| | [[File:Menstrual cycle.png|90px|left|link=Menstrual Cycle]]
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| [[Menstrual Cycle]] changes: Uterine endometrium (loss), Ovary (Follicle Development)
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| |-bgcolor="#AFEEEE"
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| | <center>2</center>
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| | [[File:Human-_menstrual_uterine_endometrium.jpg|90px|link=Menstrual_Cycle_-_Histology]]
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| | <center>3</center>
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| |-bgcolor="#AFEEEE"
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| | <center>4</center>
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| | <center>5</center>
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| | Proliferative Phase
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| | [[File:Smear- early proliferative.jpg|90px|link=Menstrual_Cycle_-_Histology]][[File:Ova41he.jpg|90px|link=Menstrual Cycle]] [[Menstrual Cycle]] changes: Uterine endometrium (proliferation), Ovary (Follicle Development)
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| |-bgcolor="#AFEEEE"
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| | <center>6</center>
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| | <center>7</center>
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| |}
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| == Week -1 ==
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| (Clinical Week 2)
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| {| class="prettytable" width=100%
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| |-bgcolor="#AFEEEE"
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| | <center>'''Day'''</center>
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| | '''Menstrual cycle'''
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| | '''Event'''
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| | <center>8</center>
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| | Proliferative Phase
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| |-bgcolor="#AFEEEE"
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| | <center>9</center>
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| | [[File:Smear-_mid-proliferative.jpg|90px|link=Menstrual_Cycle_-_Histology]] [[File:Human-_mid-proliferative_uterine_endometrium.jpg|90px|link=Menstrual_Cycle_-_Histology]] [[File:Ovary10x.jpg|90px]] [[File:Ova20he.jpg|90px]] [[Menstrual Cycle]] - Mid proliferative
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| | <center>10</center>
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| |-bgcolor="#AFEEEE"
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| | <center>11</center>
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| | <center>12</center>
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| |-bgcolor="#AFEEEE"
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| | <center>13</center>
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| | [[File:Smear-_late-proliferative.jpg|90px|link=Menstrual_Cycle_-_Histology]] [[File:Human-_late_proliferative_uterine_endometrium.jpg|90px|link=Menstrual_Cycle_-_Histology]] [[File:Menstrual cycle.png|90px|link=Menstrual Cycle]] [[Menstrual Cycle]] - Late Proliferative
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| | <center>14</center>
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| | Ovulation
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| Capacitation
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| | [[Image:Human oocyte.jpg|90px]] [[File:Follicle 001 icon.jpg|90px|link=Development_Animation_-_Ovulation]]
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| |}
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| == Week 1 ==
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| [[Week 1]] (Clinical Week 3)
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| {| class="prettytable" width=100%
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| |-bgcolor="#AFEEEE"
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| | <center>'''Day'''</center>
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| | <center>'''Stage'''</center>
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| | '''Event'''
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| | <center>1</center>
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| | Secretory PhaseStage 1
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| | [[Image:Early_zygote.jpg|90px]] [[File:Smear-_secretory.jpg|90px|link=Menstrual_Cycle_-_Histology]] [[File:Human-_secretory_uterine_endometrium.jpg|90px|link=Menstrual_Cycle_-_Histology]] Fertilization, Secretory Phase
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| |-bgcolor="#AFEEEE"
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| | <center>2</center>
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| | Stage 2
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| | [[Image:Stage2.jpg|90px]] [[File:Week1 001 icon.jpg|90px|link=Development_Animation_-_Week_1]] Morula, Blastula
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| | <center>3</center>
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| |-bgcolor="#AFEEEE"
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| | <center>4</center>
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| | Stage 3
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| | [[Image:CSt3.jpg|90px]] Blastocyst Hatching (zona pellucida lost)
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| | <center>5</center>
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| | [[File:Smear-_late_secretory.jpg|90px]] [[File:Human-_late_secretory_uterine_endometrium.jpg|90px]] Late Secretory, Blastocyst (free floating)
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| |-bgcolor="#AFEEEE"
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| | <center>6</center>
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| | Stage 4
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| | Adplantation
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| | <center>7</center>
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| | Stage 5
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| | [[File:Week2_001 icon.jpg|90px|link=Development_Animation_-_Implantation]]
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| |}
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| == Week 2 ==
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| [[Week 2]] (Clinical Week 4)
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| {| class="prettytable" width=100%
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| |-bgcolor="#AFEEEE"
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| | <center>'''Day'''</center>
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| | <center>'''Stage'''</center>
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| | '''Event'''
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| | <center>8</center>
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| | [[File:Week2_001 icon.jpg|90px|link=Development_Animation_-_Implantation]] Implantation
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| |-bgcolor="#AFEEEE"
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| | <center>9</center>
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| | <center>11</center>
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| | <center>13</center>
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| | Stage 6
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| | [[File:Chorion 001 icon.jpg|90px|link=Development Animation - Chorionic Cavity]] Chorionic Cavity
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| | <center>14</center>
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| == Week 3 ==
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| [[Week 3]] (Clinical Week 5)
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| {| class="prettytable" width=100%
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| |-bgcolor="#AFEEEE"
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| | <center>'''Day'''</center>
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| | <center>'''Stage'''</center>
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| | '''Event'''
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| | <center>15</center>
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| | <center>16</center>
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| | Stage 7
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| | [[File:Stage7-bf1.jpg|90px|link=Carnegie_stage_7]] [[File:Stage7-sem2.jpg|90px|link=Carnegie_stage_7]] [[Image:Stage7.jpg|90px|link=Carnegie_stage_7]]
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| | Stage 8
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| | [[Image:Stage8_human.jpg|90px|link=Carnegie_stage_8]] [[File:Neuralplate_001 icon.jpg|90px|link=Development Animation - Neural Plate]] [http://embryology.med.unsw.edu.au/Notes/neuron.htm Neural] neurogenesis, neural groove and folds are first seen
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| | <center>19</center>
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| | [[Image:Stage8_SEM1.jpg|90px|left]]
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| |-bgcolor="#AFEEEE"
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| | <center>20</center>
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| | Stage 9
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| | [[File:Stage9_bf2c.jpg|90px|link=Carnegie_stage_9]] [[File:Stage9_sem1b.jpg|90px|link=Carnegie_stage_9]] [http://embryology.med.unsw.edu.au/Notes/skmus6.htm Musculoskeletal] somitogenesis, first somites form and continue to be added in sequence caudally
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| [http://embryology.med.unsw.edu.au/Notes/neuron.htm Neural] the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open
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| [http://embryology.med.unsw.edu.au/Notes/ncrest.htm Neural Crest] mesencephalic neural crest is visible [http://www.ncbi.nlm.nih.gov/pubmed/17848161 PMID: 17848161]
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| | <center>21</center>
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| | [http://embryology.med.unsw.edu.au/Notes/heart.htm Heart] cardiogenesis, week 3 begins as paired heart tubes.
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| == Next ==
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| '''Finished Lab 3 !'''
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| If you have finished and would like to apply your knowledge, I have also included some [BGDlabfertilization12.htm Clinical Questions] based around this period of development.
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| If you have finished and need some more help understanding this period of development, I have included some links to [BGDlabfertilization14.htm Online References].
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| If you have finished and are interested in looking at tissues involved in this period of development, I have included some links to [BGDlabfertilization13.htm Histology Images].
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| Note that this Practical has discussed mainly development of the embryo as placental development will be covered in detail in another practical ('''Practical 8 - Placenta and Fetal Membranes''').
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| The next Practical will continue on through embryonic development ('''Practical 6 - Implantation to 8 Weeks''').
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