How quickly does chick and human embryos develop all three germ layers?
453127
Within days for the chick-2-3.  For the human, in the first 2 weeks.
786998
may be i can interpret your question this way.. why do chickens in the course of evolution formed in the way that they lay eggs, and in the same way why do humans give birth..\n\nin this case i can suggest you to think about the different modes of multiplications. chickens opted to go for the egg mode, which they found reliable and fit for survival. as evolution proceeded humans were formed and they were fit with the baby mode of multiplication.
167524
This is a highly loaded question.  At least you are up front about your intentions.\n\nThe beginning of life is subjective. Not all species follow the same rules.\n\nTake for example a jellyfish. When a jellyfish releases spermatozoa and eggs, these combine to form a planula. This is technically a living cluster of cells. It has no nervous system, no perception, no way to feed, or direct its movement. As it drifts through the water it develops further and has rudimentery chemical senes that will ultimately tell it where to settle and begin its next phase of life.\n\nA mammal is a cluster of cells a few days after conception. These are undifferentited stem cells. They have not yet begun to differentiate. An embryonic dog looks like an enbryonic whale at this stage. It is very fragile and any number of problems will cause it to self terminate and stop growing. It cannot survive without its host, and has no brain or nervous system to be conscious or self aware.  In the case of humans, whose gestation is about 42 weeks, at about week 12, the fetus now has a rudimentery circulatory system and a beating heart. This system now is responsible for delivery nutrients provided by the host (mom) and removing waste products. Any earlier and the fetus/embryo is  in early development. In many cases if you were showed any mammal at the same stage you could not tell a mouse from a dog from a human at 4, 6, or 8 weeks. Even an embryonic fish at 2 weeks looks like a human at 3 weeks. If you had to put a date on this type of question, then 12 weeks seems reasonable. Another perspective is when a fetus could survive outside the uterus, which is about 26 to 30 weeks. In some cases slightly earlier. The nervous system begins development around week 10 to 14 and for all purposes is not finished until about 2.5 years of age. The brain and spinal chord are not cohesive until about 16 t 20 weeks and there are no sensory systems or pain receptors until closer to 30 weeks. Even at birth, newbornes are only semi conscious as their brains interpret the new environment.\nIn short, you cannot accurately and faily put a number on this. Esecially when it comes to a womans right to dictate how her bidy is used. Pregnancy is much like a parasitic infection. Forcing a mother to be a host to this is as cruel as terminating a pregnancy. Prevention is far better and education and access to reliable methods and information should be everyones goals. But you cannot expect the number of pregnancy terinations to go down if you do not educate kids about sex and provide confidential access to birth control.
1042915
It depends on the species of bird. Some it may take months, some weeks, some a week, and others just a few days. It also depends on the health of the parents and baby. If the parents are healthier they can take better care of the baby so it will be healthier and can fly sooner. Also a healthier parent will teach the baby sooner and quicker. But some bird such as the ostrich, emu, and South American rhea do not fly. Sorry I was unable to be more specfic as it would have been impossible to give you an exact time period. After originally answering this question you e-mailed me saying that the birds were sparrows. Sparrows are small birds and as such, will learn to fly quickly. It may be anywhere around 6-14 days.
741835
All human beings start their lives from a single cell, called the zygote, which is formed after fertilization. The zygote divides and forms two cells; each of those cells divides again, and so on. Pretty soon, about five days after conception, there is hollow ball of about 150 cells called the blastocyst. The blastocyst is smaller than a grain of sand and contains two types of cells, the trophoblast and the inner cell mass. Embryonic stem cells are the cells that make up the inner cell mass. As embryonic stem cells can form all cell types in an adult, they are referred to as pluripotent stem cells.\n\nStem cells can also be found in very small numbers in various tissues in the adult body. For example, bone marrow stem cells are found in the marrow of the bone and they give rise to all specialized blood cell types. Adult stem cells are typically programmed to form different cell types of their own tissue; they are called multipotent stem cells. Adult stem cells have not yet been identified in all vital organs. In some tissues like the brain, although stem cells exist, they are not very active, and thus do not readily respond to cell injury or damage. Scientists are now also exploring ways in which they can induce the stem cells already present to grow and make the right cell types to replace the damaged ones. \n\nStem cells can also be obtained from sources like the umbilical cord of a newborn baby. This is an accessible source of stem cells, compared to adult tissues like the brain and bone marrow. Although scientists can grow these cells in culture dishes, they can do so only for a limited time. Recently, scientists have discovered the existence of stem cells in baby teeth and in amniotic fluid-the "water bath" that surrounds an unborn baby- and these cells may also have the potential to form multiple cell types. Research to characterize and study these cells is very promising but at a very early stage.
862639
When the egg and the spermatozoid unit, a zygote if formed.  That zygote begin to develop, neither as male or female.  Or should I say it develops as DEFAULT which is, female.  At 6 weeks of gestation, the zygote becomes an embryo and THEN, and only then, it differenciate to male or female.  \n\nAs you may know, female have XX chromosome and male have XY chromosome.  On that tiny Y chromosome there's a gene, called SRY, that tells the embryo's body to produce testicules.  If the embryo's female, then the second X commands the formation of ovaries.  Then hormones are produced, and penis or uterus appear.\n\nYour nipples are simply made before that 6th week, when the zygote is a "default female" and once they're there, they stay there, even after the testosterone hormone start being produced.
640294
Human egg = 200,000 times human sperm size
325466
About three months old
866765
The human skeleton is made of individual or joined bones (such as the skull), supported and supplemented by a structure of ligaments, tendons, muscles, cartilage and other organs.\n\nThe skeleton is not unchanging; it changes composition over a lifespan. Early in gestation, a fetus has no hard skeleton; bones form gradually during nine months in the womb. At birth, all bones will have formed, but a newborn baby has more bones than an adult. On average, an adult human has 206 bones (according to Gray's Anatomy, but the number can vary slightly from individual to individual), but a baby is born with approximately 300 bones. The difference comes from a number of small bones that fuse together during growth, such as the sacrum and coccyx of the vertebral column. An infant is born with pockets of cartilage between particular bones to allow further growth. The sacrum (the bone at the base of the spine) consists of five bones which are separated at birth but fuse together into a solid structure in later years. Growing is usually completed between ages 13 and 18, at which point the bones have no pockets of cartilage left to allow more growth.\n\nNot all bones are interconnected directly. There are 6 bones, the auditory ossicles (three on each side), in the middle ear that articulate only with each other. Another bone, the hyoid bone in the neck, does not touch any other bones in the body, and is supported by muscles and ligaments; it serves as the point of attachment for the tongue.Also some of the ribs called the floating ribs only hang by muscle and are not attached to the spine.\n\nThe longest and heaviest bone in the body is the femur and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises around 20% of the total body weight.\n\nBones of the Head\n\n    * frontal bone\n    * mandible\n    * maxilla\n    * occiptal bone\n    * parietal bone\n    * temporal bone\n    * zygomatic bone \n\nBones of the Neck and Chest\n\n    * acromion\n    * atlas\n    * cervical vertebra\n    * clavicle\n    * false rib\n    * floating rib\n    * scapula\n    * spine of scapula\n    * sternum\n    * thoracic vertebra\n    * vertibral column \n\nBones of the Abdomen\n\n    * coccyx\n    * ilium\n    * ischium\n    * lumbar vertebra\n    * sacrum\n    * vertibral column \n\nBones of the Arm\n\n    * carpus (carpals)\n    * epicondyle\n    * epitrochlea\n    * head of humerus\n    * humerus\n    * metacarpus (metacarpals)\n    * olecranon\n    * phalanx (phalanges)\n    * radius\n    * ulna \n\nBones of the Leg\n\n    * calcaneus\n    * condyle of femur\n    * femur\n    * fibula\n    * head of femur\n    * metatarsus (metatarsals)\n    * neck of femur\n    * patella\n    * phalanx (phalanges)\n    * talus\n    * tarsus (tarsals)\n    * tibia
648453
Here's the thing: I'm not an expert on fetal anatomy, but human fetuses do not have gills.  They do have tails for a few weeks, appearing around the 4th week and dispearing around the 8th.  They also have regions that are homolgous to those that develop into gills in aquatic organisms, but will form the throat of the human fetus.\n\nThe gill and tail myth was created by a man named Haeckel.  Now, Haeckel actually (whatever creationists may say) did have some good ideas, but he was a very bad scientist and had an unfortunate tendancy, especially in the later part of his career, to exagerate and even completely fabricate morphological features of embryos.\n\nHOWEVER, there _is_ an association between fetal development and evolutionary lineage.  It's just a bit more subtle than Haeckel claimed.  While embryos don't go through stages that ressemble the adult forms of their ancestors, the is a distinct correlation between the age at which the morphology of embryos of different species diverges and the evolutionary relationship between those species.  For example, human embryos and chimp embryos look really similar for a lot longer than human embryos and snake embryos.  But human and snake embryos look similar to each other long after they have completely diverged from the development of, say, a sea urchin.\n\nYour point about gills and tails isn't very accurate, and the actual connection between evolution and development is not very accesable to the non-scientist, but there is plenty of evidence for evolution that doesn't take a PhD to understand.  Here are a few points that I hope will help you next time you want to defend the theory of evolution:\n\n\n1. Vestigial structures\n\nOne example of a vestigial structure in humans is the appendix. It has no function. Removing it is harmful only in so far as any surgery is associated with certain risks (so I wouldn't have it removed just for fun, but it's because of the surgery not the absence of the appendix). An appendix is found in several mammals. In humans it is small and has no known function. In rabbits and many other herbivores it is large and is used in the digestion of cellulose.\n\nAnother example of a vestigial structure is the hipbones of snakes. Snakes evolved from quadrupeds, and some species still retain not on pelvises, but tiny protrusions of bone in a location that corresponds to the location of the legs in other quadrupeds.\n\n2. Microevolution can be observed in both a controlled laboratory setting and in nature.\n\nCreationists will tell you that microevolution does not prove macroevolution.  This is true.  However, it does provide evidence that macroevolution may be possible. Taken in the context of the other evidence for evolution, microevolution provides a mechanism by which macroevolution could occur. On its own, it doesn't prove that macroevolution occurs, but it does suggest that it is possible.\n\n3. The fossil record.\n\nAlthough the fossil record does not provide a “complete” record as fossils form only under certain conditions, it does show a gradual change in the morphology of species as well as numerous extinct species.  There are a number of methods used to date fossils, and the time period from which a fossil comes can be determined with reasonable accuracy.\n\n4. Imperfect structures (the blind spot of the mammalian eye, for example).\n\nI want to mention the bind-spot of the mammalian eye specifically because creationists often hold up the human eye up as an example of something that is too perfect to occur by "chance" (i.e. as the result of the natural selection of beneficial changes among random mutations).\n\nThere are also numerous examples where the morphology of a species is constrained by similar patterns in its ancestors (quadrupeds, vertebrates, etc.).\n\n5. Developmental biology reflects evolutionary lineage.\n\nCreationists like to bring up a man named Haeckel in response to this argument.
1111140
A team made up of a geneticist, philosopher and chicken farmer claim to have found an answer. It was the egg.\n\nPut simply, the reason is down to the fact that genetic material does not change during an animal's life.\n\nTherefore the first bird that evolved into what we would call a chicken, probably in prehistoric times, must have first existed as an embryo inside an egg.\n\nProfessor John Brookfield, a specialist in evolutionary genetics at the University of Nottingham, told the UK Press Association the pecking order was clear.\n\nThe living organism inside the eggshell would have had the same DNA as the chicken it would develop into, he said.\n\n"Therefore, the first living thing which we could say unequivocally was a member of the species would be this first egg," he added. "So, I would conclude that the egg came first."\n\nThe same conclusion was reached by his fellow "eggsperts" Professor David Papineau, of King's College London, and poultry farmer Charles Bourns.\n\nMr Papineau, an expert in the philosophy of science, agreed that the first chicken came from an egg and that proves there were chicken eggs before chickens.\n\nHe told PA people were mistaken if they argued that the mutant egg belonged to the "non-chicken" bird parents.\n\n"I would argue it is a chicken egg if it has a chicken in it," he said.\n\n"If a kangaroo laid an egg from which an ostrich hatched, that would surely be an ostrich egg, not a kangaroo egg."\n\nBourns, chairman of trade body Great British Chicken, said he was also firmly in the pro-egg camp.\n\nHe said: "Eggs were around long before the first chicken arrived. Of course, they may not have been chicken eggs as we see them today, but they were eggs."
33113
1) Because they can? :^)\n\n2) Because they like to stir up debates between creationists and evolutionists.\n\n3) Because they think it is an unanswerable riddle when it isn't.\n\nIt is an interesting question but people really don't think it through enough to show just how interesting it is.  How you answer the question reflects on your belief system, your powers of deductive reasoning, and/or your sense of humor.\n\nIf you subscribe to evolution (as I do), then it's very simple -- eggs existed long before chickens ever did.  Fish lay eggs.  Amphibians lay eggs.  Reptiles lay eggs.  Dinosaurs layed eggs.  Prototypical birds layed eggs.  Early mammals layed eggs.  (Even current mammals called monotremes lay eggs).  So obviously eggs came before chickens by this line of reasoning.\n\nFollowing evolutionary theory, genetics, and the development of embryos, one might argue that again, the egg came before the chicken because the first chicken is a genetic mutation of two chicken like parents.  So, from this line of reasoning, you might, again argue the egg if you are talking about first chicken relative to the egg.  For the first chicken had to develop inside the egg as the offspring of two chicken-like parents.\n\nBut then I realized "Hey, how did the first zygotic chicken (fertilized ovum) get inside the egg in the first place?"  If the ovum was inside an egg shell with yoke and rooster sperm is added afterward, then the egg comes first.  But what if the the ovum is fertilized first, then encased in an egg shell?  Then would not the zygotic chicken come before the egg?\n\nA quick look on Wikipedia for bird reproduction indicates that the eggs are fertilized with sperm during the descent from the ovaries through the cloaca.  If this is the case, then, again, the egg comes before the chicken.\n\nSo, I forgot to add one reason people ask this question every six minutes:\n\n4) Because they have too much time on their hands. :^)
542237
The three terrestrial classes that give rise to the four extra embryonic membranes are mammals, birds, and reptiles.\n\nThe four membranes are:\n\nChorion. The outermost of the two fetal membranes - the amnion is the innermost --. which together surround the embryo. The chorion develops villi (vascular fingers) and gives rise to the placenta. In Greek, the word "chorion" means "skin or leather."\n\nAmnion. The amnion is a membranous sac which surrounds and protects the embryo. It is developed in reptiles, birds, and mammals, which are hence called 'Amniota'; but not in amphibia and fishes, which are consequently termed 'Anamnia'.\n\nAllantois. An extra-embryonic membrane of reptiles, birds, and mammals arising as a pouch, or sac, from the hindgut. In reptiles and birds it expands greatly between two other membranes, the amnion and chorion, to serve as a temporary respiratory organ while its cavity stores fetal excretions. In mammals other than marsupials the allantois is intimately associated with the chorion, contributing blood vessels to that structure as it forms—in conjunction with the endometrium, or mucosal lining, of the uterus—the placenta. \n\nYolk Sac. A membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals and functioning as the circulatory system of the human embryo before internal circulation begins.
124945
It depends on the technique employed and the species involved.\n\nIf u carry out chromosome analysis, it's from right after fertilization.\n\nFor human, sexual differentiation occours ~6 weeks after fertilization.
155428
Skin, neural crest cells, and the neural tube are all derived from ectoderm in the embryo.\n\nMuscle cells are derived from mesoderm.
1099085
I don't think you mean development of the zygote....that is a fertilized egg.  Do you mean all the embryological stages after fertilization? There are similarities and differences in all three of these organisms' embryology.\n\nThe starfish zygote and amphibian zygote are both slimy-jelly coated and are vulnerable to fungus-bacterial infections in the water they are laid in, and vulnerable to being eaten by misc hungry aquatic organisms. These eggs are laid and fertilized as a result of external fertilization in the water.  Starfish just release the gametes in the water at a specific time in breeding season without a mating act per se.  If you are standing on a hill above a tidepool while groups of male and female starfish shed their gametes you can see the cloudiness in the water from the huge quantities of sperm and egg they release quasi-synchronistically. \n\nFrogs and other amphibians actually have a mating act called amplexus. The male does all the ribbiting to audition for the job of dna donor.  If she is ok with it, he jumps on the back of the female and he squeezes her eggs out while he releases his sperm on them. Starfish zygotes go thru many many larval stages-metamorphoses (they are part of zooplankton) and eventually mature to the adult on the tidepool rocks.  The bipinnaria larva of echinoderms are famous for their DEUTEROSTOMATE development like other vertebrates-chordates (see below)!\n\nAmphibian zygotes eventually turn into ONE aquatic tadpole larval stage which uses gills and then grow appendages-lungs to be eventually become more terrestrial.  \n\nAmphibians do not have an amnion in their eggs and are therefore anamniotes. Birds and reptiles produce "land eggs" that can resist dessication pretty well and are therefore called AMNIOTES, those who have the amniotic sac to help protect the developing embryo from the drying air on land. The amniotic fluid is in the amniotic sac. Mammals can lay eggs (monotremes) but most are placental mammals who have the amniotic sac in the uterus, but amnion and amniotic fluid with the fetus is not inside a shelled egg like the birds and reptiles.So placental mammals are the most advanced amniotes\n\nOne way these three embryo types do NOT differ: starfish and amphibian and bird are all "deuterostomate" animals.  . All animals you listed on your question are deuterostomate. Most invertebrates are "protostomes. So it is said to be an evolutionary LINK, that the echinoderm-starfish bipinnaria larva shows deuterostomate embryology! "Proto means "first," deutero means "last" and stom means "mouth." Again, both echinoderm bipinnaria larva and chordates-vertebrates (amphibians and birds) are deuterostomate. \n\nWhen embryos change from blastulas to gastrulas, there is an early indentation-invagination process of the tissue to begin the formation of endoderm and the future GI tract from MOUTH to ANUS.  In protostomates, most invertebrates except the echinoderm-starfish group, the mouth is formed first at the first indentation or invagination of blastula to become gastrula, and the anus is formed last.  \n\nIn the deuterostomates, the mouth is formed last and the anus is formed first.  The Echinoderms which includes the starfish, are deuterosomates like the chordates (bird and amphibian), including us humans. Aren't you glad you now know that your anus formed before your mouth?
1187948
It really depends on your age and development cycle....At 21 days after conception, the human heart rate begins beating a 75-80 beats per minute and accelerates linearly for the first month of beating.The human embryonic heart (EHR) begins beating at approximately 21 days after conception, or five weeks after the last normal menstrual period (LMP), which is the date normally used to date pregnancy. The human heart begins beating at a rate near the mother’s, about 75-80 beats per minute (bpm). It then accelerates linearly for the first month of beating, peaking at 165-185 bpm during the early 7th week, (early 9th week after the LMP). This acceleration is approximately 3.3 bpm per day, or about 10 bpm every three days, an increase of 100 bpm in the first month.  After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 bpm (+/-25 bpm) during the 15 week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 bpm) bpm at term. The regression formula which describes this acceleration before the embryo reaches 25 mm in crown-rump length or 9.2 LMP weeks is:\n\nAge in days = EHR(0.3)+6