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معاً خطوة بخطوة لمتابعة الجنين لحتى يولد


معاً خطوة بخطوة لمتابعة الجنين لحتى يولد

بسم الله الرحمن الرحيم ...

إخوتي وأخواتي الكرام ..

حبيت نتابع معاكم بشكل مستمر مادة الجنين الممتعة والتي يحاول دكاترتها من جعلها مادة حفظ وانما هي تماماً مادة استنتاجات وفهم ...

هذا رأيي الهاص ما بعرف عنكم شو رأيكم ...

منطلقاً من هذا المبدأ أنا عم بجمع مواضيعنا بالجنين من مواقع مختلفة ندرس فيها ( وباللغة الانكليزية مادة الجنين ) موضحةً بالصور اللزمة لفهم المادة حتى باللغة العربية ...

وبسم الله نبدأ :

ملاحظة : رح أبدأ من عملية التلقيح لأنه أكيد خطوات تشكل الأعراس الجميع بيعرفها لذلك رح أبدأ منطلقاً من عملية التلقيح

FERTILIZATION

Fertilization of the Ovum

FIG. 8– The process of fertilization in the ovum of a mouse. (After Sobotta.)

Fertilization consists in the union of the spermatozoön with the mature ovum (Fig. 8). Nothing is known regarding the fertilization of the human ovum, but the various stages of the process have been studied in other mammals, and from the knowledge so obtained it is believed that fertilization of the human ovum takes place in the lateral or ampullary part of the uterine tube, and the ovum is then conveyed along the tube to the cavity of the uterus—a journey probably occupying seven or eight days and during which the ovum loses its corona radiata and zona striata and undergoes segmentation. Sometimes the fertilized ovum is arrested in the uterine tube, and there undergoes development, giving rise to a tubal pregnancy; or it may fall into the abdominal cavity and produce an abdominal pregnancy. Occasionally the ovum is not expelled from the follicle when the latter ruptures, but is fertilized within the follicle and produces what is known as an ovarian pregnancy. Under normal conditions only one spermatozoön enters the yolk and takes part in the process of fertilization. At the point where the spermatozoön is about to pierce, the yolk is drawn out into a conical elevation, termed the cone of attraction. As soon as the spermatozoön has entered the yolk, the peripheral portion of the latter is transformed into a membrane, the vitelline membrane which prevents the passage of additional spermatozoa. Occasionally a second spermatozoön may enter the yolk, thus giving rise to a condition of polyspermy: when this occurs the ovum usually develops in an abnormal manner and gives rise to a monstrosity. Having pierced the yolk, the spermatozoön loses its tail, while its head and connecting piece assume the form of a nucleus containing a cluster of chromosomes. This constitutes the male pronucleus, and associated with it there are a centriole and centrosome. The male pronucleus passes more deeply into the yolk, and coincidently with this the granules of the cytoplasm surrounding it become radially arranged. The male and female pronuclei migrate toward each other, and, meeting near the center of the yolk, fuse to form a new nucleus, the segmentation nucleus, which therefore contains both male and female nuclear substance; the former transmits the individualities of the male ancestors, the latter those of the female ancestors, to the future embryo. By the union of the male and female pronuclei the number of chromosomes is restored to that which is present in the nuclei of the somatic cells


FIG. 9– First stages of segmentation of a mammalian ovum. Semidiagrammatic. (From a drawing by Allen Thomson.) z.p. Zona striata. p.gl. Polar bodies. a. Two-cell stage. b. Four-cell stage. c. Eight-cell stage. d, e. Morula stage.

اليوم خلصنا ولكم كل يوم مسلسلا بسيط عن حياتنا بس قبل ما نولد ..

bEFORE WE ARE BORN >>>

شدوا الحيل وأطلب منكم التشجيع لأنه الكميات قصيرة والفهم كتير ضروري ..

تحيتي الكم والسلام خير الختام ...

أخوكم

DR-MAK

:o :o :o

لو علِمْتمْ كَيْفَ يُدَبّرُ الرّبُ أمُورَكمْ لذابَتْ قلوبُكمْ مِنْ مَحَبته ♥ .

by


ولك شو هاد يا معلم Shocked Shocked Shocked

والله متعب حالك

بس من جد دكتور بن دكتور 8) 8) 8)

يا أحلى mak

بعدين شو هالصورة الحلوة يا

مابتستحي تحط صورة وانا لهلق ما حطيت وحدة Eye-wink Eye-wink Eye-wink

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Dr-Murk
السنة السادسة


حلوين كتير الله يعطيك العافية

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jemeni_boy
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الحقيقة موقع الكلية الجديد net. عم يقدم خدمة المكتبة الإلكترونية و فيكم تلاقو مواقع كتير حلوة عليه بالنسبة لأغلب المواد و مششششششششكور MAK على كل حال

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I.A


شكرا يا احلى موكو بالتاريخ Laughing out loud Laughing out loud Laughing out loud

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Dr_Hero
طبيب مقيم

Quote:
الحقيقة موقع الكلية الجديد net. عم يقدم خدمة المكتبة الإلكترونية

ممكن تعطينا الرابط

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person
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الرابط :

www.medamas.org

بس على فكرة هالحديث عن تطور الجنين مو من هالمكاتب اللي شرّف عماد وحكى عنها هي مجمعها من عدة مصادر ومركبها عرفت شلون ؟؟

DR-MAK


و اذا مجمعها ما حلوة....

(عم امزح)(اهههههه فهمت علي)
حاكينا يا ماك

nortex=1's picture
nortex=1
السنة الرابعة


. Segmentation of the Fertilized Ovum

The early segmentation of the human ovum has not yet been observed, but judging from what is known to occur in other mammals it may be regarded as certain that the process starts immediately after the ovum has been fertilized, i. e., while the ovum is in the uterine tube. The segmentation nucleus exhibits the usual mitotic changes, and these are succeeded by a division of the ovum into two cells of nearly equal size. 5 The process is repeated again and again, so that the two cells are succeeded by four, eight, sixteen, thirty-two, and so on, with the result that a mass of cells is found within the zona striata, and to this mass the term morula is applied (Fig. 9). The segmentation of the mammalian ovum may not take place in the regular sequence of two, four, eight, etc., since one of the two first formed cells may subdivide more rapidly than the other, giving rise to a three-or a five-cell stage. The cells of the morula are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast, which does not contribute to the formation of the embryo proper, and an inner cell-mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a vesicle, the blastodermic vesicle (Fig. 10). The inner cell-mass remains in contact, however, with the trophoblast at one pole of the ovum; this is named the embryonic pole, since it indicates the situation where the future embryo will be developed. The cells of the trophoblast become differentiated into two strata: an outer, termed the syncytium or syncytiotrophoblast, so named because it consists of a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells; and an inner layer, the cytotrophoblast or layer of Langhans, in which the cell outlines are defined. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of the chorion and play an important part in the development of the placenta. On the deep surface of the inner cell-mass a layer of flattened cells, the entoderm, is differentiated and quickly assumes the form of a small sac, the yolk-sac. Spaces appear between the remaining cells of the mass (Fig. 11), and by the enlargement and coalescence of these spaces a cavity, termed the amniotic cavity (Fig. 12), is gradually developed. The floor of this cavity is formed by the embryonic disk composed of a layer of prismatic cells, the embryonic ectoderm, derived from the inner cell-mass and lying in apposition with the entoderm.

FIG. 10– Blastodermic vesicle of Vespertilio murinus. (After van Beneden.)

FIG. 11– Section through embryonic disk of Vespertilio murinus. (After van Beneden.)

FIG. 12– Section through embryonic area of Vespertilio murinus to show the formation of the amniotic cavity. (After van Beneden.)

The Primitive Streak; Formation of the Mesoderm.—The embryonic disk becomes oval and then pear-shaped, the wider end being directed forward. Near the narrow, posterior end an opaque streak, the primitive streak (Figs. 13 and 14), makes its appearance and extends along the middle of the disk for about one-half of its length; at the anterior end of the streak there is a knob-like thickening termed Hensen’s knot. A shallow groove, the primitive groove, appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, the blastophore, with the yolk-sac. The primitive streak is produced by a thickening of the axial part of the ectoderm, the cells of which multiply, grow downward, and blend with those of the subjacent entoderm (Fig. 15). From the sides of the primitive streak a third layer of cells, the mesoderm, extends lateralward between the ectoderm and entoderm; the caudal end of the primitive streak forms the cloacal membrane

FIG. 13– Surface view of embryo of a rabbit. (After Kölliker.) arg. Embryonic disk. pr. Primitive streak

The extension of the mesoderm takes place throughout the whole of the embryonic and extra-embryonic areas of the ovum, except in certain regions. One of these is seen immediately in front of the neural tube. Here the mesoderm extends forward in the form of two crescentic masses, which meet in the middle line so as to enclose behind them an area which is devoid of mesoderm. Over this area the ectoderm and entoderm come into direct contact with each other and constitute a thin membrane, the buccopharyngeal membrane, which forms a septum between the primitive mouth and pharynx. In front of the buccopharyngeal area, where the lateral crescents of mesoderm fuse in the middle line, the pericardium is afterward developed, and this region is therefore designated the pericardial area. A second region where the mesoderm is absent, at least for a time, is that immediately in front of the pericardial area. This is termed the proamniotic area, and is the region where the proamnion is developed; in man, however, a proamnion is apparently never formed. A third region is at the hind end of the embryo where the ectoderm and entoderm come into apposition and form the cloacal membrane. 3
The blastoderm now consists of three layers, named from without inward: ectoderm, mesoderm, and entoderm; each has distinctive characteristics and gives rise to certain tissues of the body. 6

FIG. 14– Surface view of embryo of Hylobates concolor. (After Selenka.) The amnion has been opened to expose the embryonic disk.

FIG. 15– Series of transverse sections through the embryonic disk of Tarsius. (After Hubrecht.) Section I passes through the disk, in front of Hensen’s knot and shows only the ectoderm and entoderm. Sections II, III, and IV pass through Hensen’s knot, which is seen in V tapering away into the primitive streak. In III, IV, and V the mesoderm is seen springing from the keel-like thickening of the ectoderm, which in III and IV is observed to be continuous into the entoderm.

Ectoderm.—The ectoderm consists of columnar cells, which are, however, somewhat flattened or cubical toward the margin of the embryonic disk. It forms the whole of the nervous system, the epidermis of the skin, the lining cells of the sebaceous, sudoriferous, and mammary glands, the hairs and nails, the epithelium of the nose and adjacent air sinuses, and that of the cheeks and roof of the mouth. From it also are derived the enamel of the teeth, and the anterior lobe of the hypophysis cerebri, the epithelium of the cornea, conjunctiva, and lacrimal glands, and the neuro-epithelium of the sense organs. 5

Entoderm.—The entoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube excepting part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm), the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas, the epithelium of the auditory tube and tympanic cavity, of the trachea, bronchi, and air cells of the lungs, of the urinary bladder and part of the urethra, and that which lines the follicles of the thyroid gland and thymus.

FIG. 16– A series of transverse sections through an embryo of the dog. (After Bonnet.) Section I is the most anterior. In V the neural plate is spread out nearly flat. The series shows the uprising of the neural folds to form the neural canal. a. Aortæ. c. Intermediate cell mass. ect. Ectoderm. ent. Entoderm. h, h. Rudiments of endothelial heart tubes. In III, IV, and V the scattered cells represented between the entoderm and splanchnic layer of mesoderm are the vasoformative cells which give origin in front, according to Bonnet, to the heart tubes, h; l.p. Lateral plate still undivided in I, II, and III; in IV and V split into somatic (sm) and splanchnic (sp) layers of mesoderm. mes. Mesoderm. p. Pericardium. so. Primitive segment

Mesoderm.—The mesoderm consists of loosely arranged branched cells surrounded by a considerable amount of intercellular fluid. From it the remaining tissues of the body are developed. The endothelial lining of the heart and blood-vessels and the blood corpuscles are, however, regarded by some as being of entodermal origin. 7
As the mesoderm develops between the ectoderm and entoderm it is separated into lateral halves by the neural tube and notochord, presently to be described. A longitudinal groove appears on the dorsal surface of either half and divides it into a medial column, the paraxial mesoderm, lying on the side of the neural tube, and a lateral portion, the lateral mesoderm. The mesoderm in the floor of the groove connects the paraxial with the lateral mesoderm and is known as the intermediate cell-mass; in it the genito-urinary organs are developed. The lateral mesoderm splits into two layers, an outer or somatic, which becomes applied to the inner surface of the ectoderm, and with it forms the somatopleure; and an inner or splanchnic, which adheres to the entoderm, and with it forms the splanchnopleure (Fig. 16). The space between the two layers of the lateral mesoderm is termed the celom. 8
Note 5. In the mammalian ova the nutritive yolk or deutoplasm is small in amount and uniformly distributed throughout the cytoplasm; such ova undergo complete division during the process of segmentation, and are therefore termed holoblastic. In the ova of birds, reptiles, and fishes where the nutritive yolk forms by far the larger portion of the egg, the cleavage is limited to the formative yolk, and is therefore only partial; such ova are termed meroblastic. Again, it has been observed, in some of the lower animals, that the pronuclei do not fuse but merely lie in apposition. At the commencement of the segmentation process the chromosomes of the two pronuclei group themselves around the equator of the nuclear spindle and then divide; an equal number of male and female chromosomes travel to the opposite poles of the spindle, and thus the male and female pronuclei contribute equal shares of chromatin to the nuclei of the two cells which result from the subdivision of the fertilized ovum. [back]
Note 6. The mode of formation of the germ layers in the human ovum has not yet been observed; in the youngest known human ovum (viz., that described by Bryce and Teacher), all three layers are already present and the mesoderm is split into its two layers. The extra-embryonic celom is of considerable size, and scattered mesodermal strands are seen stretching between the mesoderm of the yolk-sac and that of the chorion.

وهيك خلصنا الحلقة التانية سهلة وحلوة كتير على مستوى الخط البدائي والتحريض المشكل للحبل الظهري لا تقولوا كتير حلوة ...

تحيتي الكم وستنوا الحلقة القادمة ...

Laughing out loud Laughing out loud Laughing out loud

DR-MAK


اي شو هاد والله شي حلو شكرا كتير

a.ahmad's picture
a.ahmad
السنة السادسة


لك آآآآآآخ خ خ خ خ

ووأووف

شو هال الحلو

Dr-Murk's picture
Dr-Murk
السنة السادسة


أنتم الحلوين أختي وأشكر لكم المتابعة وبتمنى تدرسوهم لبانه رح تبين فائدتهم مو هلأ بس بينما لما تقدم الستيبّات ان شاء الله السنة السادسة بكرة باذن الله رح نزل الحلقة الثالثة الجنين ...

ادعولنا بظهر الغيب فان شاء الله مقبول ...

أخوكم محمد

DR-MAK


- شو متابعة ما متابعة سرقتلنا اسم الفريق يا محمد Laughing out loud Laughing out loud Laughing out loud

- يا عيني عليك ، مو معقول ، شو الحلا على قولة "بردويل" المعلق اللبناني ، يعني كبسة دنك و عالطاير و أحلى من مايكل جوردان بحد ذاته ..

- فعلاً موهبة من بدايتك 8) ، بعدين لا تخاف إن دعولك و إن ما دعولك أجرك كبير إن شاء الله لأنو فعلاً شيء جمييييييييل ..

- أخي الله يرضى عليك كلو إلا شباب الزبداني ليش عم تبخعوا لعماد ... هو صحيح بيستاهل ينبخع بس لا تخاف بكرة بشوفو و بعملو اللازم .. Twisted Evil

- طلب : ممكن إحتفظ بهذا الشرح و الصور شخصياً ..

- سؤال : بالصيف فيك تفتح إنترنت و لح تدل تفوت على حكيم و لا لأ ؟؟

KMG's picture
KMG
طبيب مقيم


الله يعطيك عافية على هل المعلومات وشوي ثانية لحا تكسح moore المفضل عندي.

Dr.FreeMan's picture
Dr.FreeMan
بعد التخرج


حلقتنا اليوم عن مستوى بسيط وهو مستوى الانبوب العصبي تشكله تمايزه ... ما يعطي ؟

كل هذه الأامور ان شاء الله ستفهم من الاسطر التالية

FIG. 17– Human embryo—length, 2 mm. Dorsal view, with the amnion laid open. X 30. (After Graf Spee.)

In front of the primitive streak two longitudinal ridges, caused by a folding up of the ectoderm, make their appearance, one on either side of the middle line (Fig. 16). These are named the neural folds; they commence some little distance behind the anterior end of the embryonic disk, where they are continuous with each other, and from there gradually extend backward, one on either side of the anterior end of the primitive streak. Between these folds is a shallow median groove, the neural groove (Figs. 16, 17). The groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, the neural tube or canal (Fig. 18), the ectodermal wall of which forms the rudiment of the nervous system. After the coalescence of the neural folds over the anterior end of the primitive streak, the blastopore no longer opens on the surface but into the closed canal of the neural tube, and thus a transitory communication, the neurenteric canal, is established between the neural tube and the primitive digestive tube. The coalescence of the neural folds occurs first in the region of the hind-brain, and from there extends forward and backward; toward the end of the third week the front opening (anterior neuropore) of the tube finally closes at the anterior end of the future brain, and forms a recess which is in contact, for a time, with the overlying ectoderm; the hinder part of the neural groove presents for a time a rhomboidal shape, and to this expanded portion the term sinus rhomboidalis has been applied (Fig. 18). Before the neural groove is closed a ridge of ectodermal cells appears along the prominent margin of each neural fold; this is termed the neural crest or ganglion ridge, and from it the spinal and cranial nerve ganglia and the ganglia of the sympathetic nervous system are developed. By the upward growth of the mesoderm the neural tube is ultimately separated from the overlying ectoderm.

FIG. 18– Chick embryo of thirty-three hours’ incubation, viewed from the dorsal aspect. X 30. (From Duval’s “Atlas d’Embryologie.”)

The cephalic end of the neural groove exhibits several dilatations, which, when the tube is closed, assume the form of three vesicles; these constitute the three primary cerebral vesicles, and correspond respectively to the future fore-brain (prosencephalon), mid-brain (mesencephalon), and hind-brain (rhombencephalon) (Fig. 18). The walls of the vesicles are developed into the nervous tissue and neuroglia of the brain, and their cavities are modified to form its ventricles. The remainder of the tube forms the medulla spinalis or spinal cord; from its ectodermal wall the nervous and neuroglial elements of the medulla spinalis are developed while the cavity persists as the central canal.

شفتم شو حلقتنا اليوم قصيرة هي للعالم اللي عندها مذاكرات خلال هالأيام ..

تعليقات خفيفة :

أخي Dr.FreeMan
الله يعافيك يا رب ما في شي من واجبكم ...

أخي KMG :
ان شاء الله أنتم من أمامنا ونحن من ورائكم ... ادعولنا بالنجاح الدنيوي والأخروي ...

:o :o :o

انتظرونا بحلقة بكرة ان شاء الله ...

أخوكم : محمد

Laughing out loud Laughing out loud Laughing out loud

DR-MAK


جزاك الله خيراً يا : Dr.MAK..... :o

cute Loly


أحلى دعوة بتنسمع اختي لبنه ...

سويةً ان شاء الله ...

DR-MAK


يسلموا هل الإيدين , و الله يقدرنا نردلك هل الجميل . Razz

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FALAFEL
السنة الثالثة


يسلم ايديك يا ماك

Quote:
جزاك الله خيرا يا : DR.MAK

nortex=1's picture
nortex=1
السنة الرابعة


له العفو أخي نورس ولو تكرم عينك ...

حبيبي نورس الله يخليك انا بعتذر عن التقصير معكم من ناحية الروحات والطلعات بس جد مقصرين ولازم نلحق متل ما كنا قبل مذاكرة الجنين ..

ادعولنا وبعتذر مرة تانية ...

DR-MAK


انتظروا بكرة ان شاء الله الحلقة القادمة من تطور الجنين كتير كتير حلوة ومظبطة تمام ...

DR-MAK


تفضلوا يا جماعة متل ما وعدتكم حلقة اليوم وهي كتير كتير بسيطة وقصيرة :

تطور الحبل الظهري وبهالسطور البسيطة رح تفهموا كل شي عنها

ملاحظة : يا ليت لأجل الفائدة تحطوا جنبكم دفتر صغير تكتبوا فيه الكلمات الجديدة اللي تعلمتوها ...

The Notochord

The notochord (Fig. 19) consists of a rod of cells situated on the ventral aspect of the neural tube; it constitutes the foundation of the axial skeleton, since around it the segments of the vertebral column are formed. Its appearance synchronizes with that of the neural tube. On the ventral aspect of the neural groove an axial thickening of the entoderm takes place; this thickening assumes the appearance of a furrow—the chordal furrow—the margins of which come into contact, and so convert it into a solid rod of cells—the notochord—which is then separated from the entoderm. It extends throughout the entire length of the future vertebral column, and reaches as far as the anterior end of the mid-brain, where it ends in a hook-like extremity in the region of the future dorsum sellæ of the sphenoid bone. It lies at first between the neural tube and the entoderm of the yolk-sac, but soon becomes separated from them by the mesoderm, which grows medial-ward and surrounds it. From the mesoderm surrounding the neural tube and notochord, the skull and vertebral column, and the membranes of the brain and medulla spinalis are developed.

FIG. 19– Transverse section of a chick embryo of forty-five hours

:o :o :o

DR-MAK


شكلاا كتير إلك أخي ماك و الله إنك بطل . و أنا بصراحة ما عبلحق اقرأ حلقة إلا بتكون نزلت إلى بعدا . عنجد الله يعطيك العافية

أخوك HaddosH

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haddosh
بعد التخرج


وشو يعني ولحنا
كمان بنقول

الله يجزيك الخير Laughing out loud Laughing out loud

ماحدا احسن من حدا Eye-wink Eye-wink

Dr-Murk's picture
Dr-Murk
السنة السادسة


أخي هدوش : مورك :

شكرا كتير الكم عالتشجيع وتكرم عينكم طالما الفائدة موجودة فانا جاهز لكل شي ...

مرة أخرى بكرر : أي مساعدة بتحتاجوها فأنا جاهز ..

واللي بيحب يشوفني بالكلية كمان جاهز لشرح أي غموض وشكرا كتير الكم

DR-MAK


يا سيدي انت واحد كفو..
الله يبدلك بكل كلمة بتنفعنا فيها بعلامة يوم الفحص الأخير..

حلوة هي الدعوة؟.....

dr-dark angel's picture
dr-dark angel
السنة السادسة


شكراص عدعونك وفهمت شو قصدك يا معلم ...!!!!

على كل أنا عم أنزل هالمواضيع للي بدو يفهم ....

وان شاء الله الكل بيستفيد وبما فيهم أنت !!!

[img][/img]

DR-MAK


Toward the end of the second week transverse segmentation of the paraxial mesoderm begins, and it is converted into a series of well-defined, more or less cubical masses, the primitive segments (Figs. 18, 19, 20), which occupy the entire length of the trunk on either side of the middle line from the occipital region of the head. Each segment contains a central cavity—myocœl—which, however, is soon filled with angular and spindle-shaped cells.

FIG. 20– Dorsum of human embryo, 2.11 mm. in length. (After Eternod.)

The primitive segments lie immediately under the ectoderm on the lateral aspect of the neural tube and notochord, and are connected to the lateral mesoderm by the intermediate cell-mass. Those of the trunk may be arranged in the following groups, viz.: cervical 8, thoracic 12, lumbar 5, sacral 5, and coccygeal from 5 to 8. Those of the occipital region of the head are usually described as being four in number. In mammals primitive segments of the head can be recognized only in the occipital region, but a study of the lower vertebrates leads to the belief that they are present also in the anterior part of the head, and that altogether nine segments are represented in the cephalic region.

بعتذر عن قصر حلقة اليوم بس لأجل مذاكرة التشريح غداً ...

Laughing out loud Laughing out loud Laughing out loud

DR-MAK


شكرا لجهودك الفعالة أخي Dr_MAK
والله يجزاك الخير
ويوفقنا بمذاكرة التشريح بكرا Shocked Shocked

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Dr_Hero
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The embryo increases rapidly in size, but the circumference of the embryonic disk, or line of meeting of the embryonic and amniotic parts of the ectoderm, is of relatively slow growth and gradually comes to form a constriction between the embryo and the greater part of the yolk-sac. By means of this constriction, which corresponds to the future umbilicus, a small part of the yolk-sac is enclosed within the embryo and constitutes the primitive digestive tube

FIG. 21– Section through the embryo which is represented in Fig. 17

The embryo increases more rapidly in length than in width, and its cephalic and caudal ends soon extend beyond the corresponding parts of the circumference of the embryonic disk and are bent in a ventral direction to form the cephalic and caudal folds respectively (Figs. 26 and 27). The cephalic fold is first formed, and as the proamniotic area (page 47) lying immediately in front of the pericardial area (page 47) forms the anterior limit of the circumference of the embryonic disk, the forward growth of the head necessarily carries with it the posterior end of the pericardial area, so that this area and the buccopharyngeal membrane are folded back under the head of the embryo which now encloses a diverticulum of the yolk-sac named the fore-gut. The caudal end of the embryo is at first connected to the chorion by a band of mesoderm called the body-stalk, but with the formation of the caudal fold the body-stalk assumes a ventral position; a diverticulum of the yolk-sac extends into the tail fold and is termed the hind-gut. Between the fore-gut and the hind-gut there exists for a time a wide opening into the yolk-sac, but the latter is gradually reduced to a small pear-shaped sac (sometimes termed the umbilical vesicle), and the channel of communication is at the same time narrowed and elongated to form a tube called the vitelline duct.

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DR-MAK


ابو شريك انا رح اكتب هالرد هون و ما رح ابعتلك يا خاص ..بعرفك بدك الفائدة للجميع بس مانك حاسس انك عم تضيع و قت كتير بالكتابة...والله حرام يعني يعطيك العافية بس انا ما برضى انك تضيع وقتك على حساب دراستك..

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