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EXTRA-EMBRYONIC MEMBRANES in CHICK Yolksac, Yolk Sac (Gut); Amnion and Chorion (Body Wall) and Allantois (Primitive Bladder). Al

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EXTRA-EMBRYONIC MEMBRANES in CHICK Yolksac, Yolk Sac (Gut); Amnion and Chorion (Body Wall) and Allantois (Primitive Bladder). Al EXTRA-EMBRYONIC MEMBRANES IN CHICK The history of the foetal-membranes in chick is with the of an correlated presence anormous mass of and an yolk embryonic life within a shell. the a spent Although, original blastoderm is small disc, it spreads by peripheral and covers the surface of the But growth eventually entire egg. only the most central region is directly concerned with the formation of the The is embryo proper. remaining blastoderm extra-embryonic and it is this portion that furnishes the embryonic or foetal membranes. These which arise are auxillary organs partly for protection of the embryo, and more especially to provide for nutrition, respiration and excretion until the independent existence is attained. its The fundamental set of embryonic membranes in chick and other amniotes includes the yolksac, amnion, chorion (serosa in mammals) and allantois which have evolved in relation to the structures that were already in existence following Yolk sac (gut); amnion and chorion (body wall) and allantois (primitive bladder). All these extra-embryonic membranous sacs are composite structures and involve two germ The layers. amnion and chorion are composeed of extra-embryonic ectoderm and somatic layer of mesoderm (somatopleure) whiler the yolk sac and allantois are composed of extra-embryonic endoderm and splanchnic layer of mesoderm (splanchnopleure), separated by a space which is extra-embryonic coelom. These components are continuous with their counterparts in the embryonic teritory of the blastoderm.. 1. Yolk Sac Yolk sac is the first of the extra-embryonic membranes to make its appearance. The splanchnopleure of the chick, instead of forming a closed gut, grows over the yolk surface as extra-embryonic extension and eventually encloses the entire yok to form the yolk sae, The primitive gut has a cellular wall dorsally only, with the yolk acting as a temporary floor (Fig. 24). However, concomitantly with the spreading of extra-embryonic splanchnopleure about the yolk, the intra-embryonic splanchnopleure is undergoing a series of changes which result in the establishment ofa completely walled gut in the body of embryo. Part of the gut which stil remains open to the yolk is known as the midgut. But by the progress of the sub-cephalic and sub-caudal folds, the foregut and hindgut increase in extent at the expense of midgut. The midgut is finally diminished until it opens ventrally by a small aperture which flares out like an inverted funnel into the yolk sac. This opening is the yolk duct and its wall constitutes the narrow yolk stalk through which the walls of yolk sac are still continuous with the wall of the gut. As the neck of the yolk sac is constricted, the omphalomesenteric (vitelline) arteries and omphalomesenteric (vitelline) veins are brought together and trasverse the yolk stalk side by side. The vascular network in the splanchnopleure of the yolk sac encompass it. Apparently no yolk passes directly through the yolk duct into the intestine, rather endodermal cells lining the yolk sac secrete some approrpriate digestive enzymes which make the yolk soluble at the spot. This soluble material is absorbed through the lining of the vitelline blood vessels to the circulating blood by which it is to cariedinto the all parts of the growing embryo. In older embryos the epithelium of the yolk sac penetrates deep yolk and formsa series of foldings which greatly increase its surface area for absorption. development, albumen loses water, becomes more viscid and rapidly decreases in bulk. The Duri allante an pur the allantois, extra-embryonic of structure, forces the albumen towards the distal end of the grOwthwdhksac (Fig. 33.2:33.25B) and the albumen is eventually compressed by the yolk sac. The yolk sac absorbs Ik way of the circulation ansfers by extra-embryonic to the embryo. and ofinc gnd the end of period, on the owards incubation usually 19th day the remains of yolk sac are pulled into 10vitycavity ofof the. the embryo through the naval and the remaining contents and wall of the yolk sac belly the disappear, their absorption is completed in the first six days after hatching and the body wall rapidly closes behindit. and Chorion Amnion 2. rhe amnion and chochorion are so closely associated during their origin that they are considered The amnion Roth are derived from the The together. extra-embryonic somatopleure. first indication of appears at about 30 hours of incubation. The head of the embryo sinks slightly into theamnion yolk form TOhethe samesamne time extra-embryonic somatopleure in front of the head forms a fold, the head fold of the d at In dorsal aspect the margin of this fold is crescentic in shape with its concavity directed towards amnion.i ae head of the embryo. As the embryo increases in length, its head grows anteriorly into the amniotic thehead. d At the same time growth in the somatopleure makes the head-fold over the head of the embryo. By fold. ontinuation of these two growth processes the head soon comes to lie in a double-walled pocket of extra-embryonicssomatopleure which covers the head like:a cap. On the third day, the tail-fold of the emnion springs up similarly from the margin of a posterior limiting fold. It grows in the opposite direction towards head. Its concavity is directed anteriorly and progression towards head. the The lateral folds of amnion develop dorso-medianlly. The head and tail folds grow to meet over are soon formed lateral folds of the amnion to hack of the embryo and joined mid-dorsally by similarly of stalk. At the enclose the embryo in two sheets of somatopleure from all sides except the region yolk a called sero-amniotic or Doint where the folds meet, they become fused in scar-like thickening raphe later and at a time sero-amniotic connection. Since, the tail-fold of amnion arises at the much stage of the of the tail fold is smaller, when the head-fold has already grown beyond the middle length embryo, The amniotic folds invovle and sero-amniotic connection is behind the middle of the embryo. essentially the doubling of somatopleure upon itself. of ectoderm and an inner lining of somatic The outer wall of the somatopleure has an outer layer The inner wall of the fold has the layers reversed, the two form the chorion. mesoderm, layers together The inner and the two together form amnion. ie, its outer layer is of somatic mesoderm and of ectoderm is a of the extra-embryonic chorion and amnion (sero-amniotic cavity) part cavity between (serosa) fold it formed in front of head of embryo (the head ), cOelom. when amniotic fold is first However, from the start consist of ectodermal layer. The lateral folds consists only of part of the extra-embryonic the mesoderm. Meseoderm also secondarily penetrates CAUTa-embryonic ectoderm and extra-embryonic of the fold becomes uniform throughout. aierior portion of the fokd, so composition over and around the embryo enclose an amniotic cavity, AS they the folds of amnion meet, slit between the embryo and the is at first a very narrow lined ectoderm. This cavity the amniotic pletely by secreted into the cavity by epithelium, soon a fluid is of the amniotic fold, but in the cavity, connected to wall lies in a pond, floating freely distends it. Thus, embryo, in a way umbilical cora. Ad-embryonic parts only by the Neural tube Notochord Ectoderm Somatic mesoderm Amniotic Gut cavity Amnion Chorion (Serosa) Allantoic Sero-amniotic vesicle cavity Yolk sac (Extraembryonic) cavity Splanchnic mesoderm Septa in Endoderm yoik sac Vitelline membrane Yolk A Gut Allantoic Chorio-allantoic vesicle membrane (Serosa and Allantois allantois) Amnion Amniotic cavity Allantoic Extra- stalk embryonic coelom Yolk sac Spianchnic mesoderm Endoderm Yolk 3 Fig. 33.25 Stages in the development of foetal membranes. the îluid to dissociation. Besides this, o The salty amniotic fluid does all that is necessary prevent from desiccation. Unstripe efficient shock absorber and protects embryo possible amniotic cavity is an rocking waves of contraction, tne the mesoderm on the fifth day and produce muscle fibres develop in from adhesion to various embryonie membranes or to itselfor frietno embryo genily, thus, preventing its against it. continues to and orionrion (serosa) grow keeps pace with the growing blastoderm and exocoel. It (serosa) sac and ChOnd the yolk envelope it. The albumen sac is also surrounded by folds of establio chorion. grows its ws afterer its establishment, develops between chorion and amnion. Thus, chorion encompasses Allantois, other extraembryonic membranes. over the Aabryo and aall It forms an additional protective umbrella embryo. effect. It e Accordingrding to BaBalinsky (1970), formation of amniotic cavity has a slightly negative the embryo fronfrom the surface of egg and forms the source of oxygen. removesthe 3. Allantois It evolved in and birds as a temporary sac for urinary storage. This accessory organ primarily reptiles and serosa because it arises within the of the embryo. Its proximal portion ars from amnion body however, carried outside the remains intra-embry throughout development. Its distal portion, ins intra-embryonic is membranes. coelom becomes with other extra-embryonic confinesfines ofofint intra-embryonic and assoçiated of out late on the third day of incubation as a diverticulum splanchnopleure fromn Allantois grows allantois in front of the anal plate. During fourth day of development ntral wall of the hindgut just ventr coelom. Its narrow proximal portion, of the body of into the extra-embryonic shes out the embryo and the distal portion caudal to the stalk, is known as allantoic stalk enlarged is parallel and yolk of its terminal hich in allnatois distends making the appearance ac allantoic vesicle. Fluid accumulating portion balloon-like.
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