21. What are histogenesis and organogenesis?
Histogenesis is the process of tissue formation in the embryonic development. Organogenesis is the process of organ formation. Before histogenesis and organogenesis the primitive embryonic structures have been already formed: germ layers, neural tube, notochord, coeloms, somites.
22. What are twins? Genetically what are the two types of twins that can be generated?
Twins are simultaneously generated (within the mother’s uterus) offspring. Twins classify according to zygosity as monozygotic or as dizygotic twins.
Monozygotic twins, also known as identical twins, are those originated from one single fertilized ovum (therefore from one single zygote); monozygotic twins are genetically identical, i.e., they have identical genotypes and are necessarily of the same sex. Dizygotic twins, also known as fraternal twins, are those generated from two different ova fecundated by two different sperm cells; so they are not genetically identical and they are not necessarily of the same sex.
Image Diversity: twins
23. What is polyembryony?
Polyembryony is the phenomenon in which a single embryo in its initial embryonic stage divides itself forming many new individuals of the same sex and genetically identical. This is the way, for example, in which reproduction takes place in armadillos of the genus Dasypus. Polyembryony is an example of natural “cloning”.
24. What are extra embryonic membranes?
Extra embryonic membranes are membranous structures that appear paralleling the embryo and play important roles in the embryonic development. They form from the embryo but do not become part of the individual organism after its birth.
25. What are the extra embryonic membranes present in vertebrates?
The extra embryonic membranes that may be present in vertebrates are the yolk sac, the amnion, the chorion, the allantois and the placenta.
26. Are the extra embryonic membranes the same in all vertebrates?
The presence of each extra embryonic membrane varies according to the vertebrate class.
In fishes and amphibians, only the yolk sac is present. In reptiles and aves besides the yolk sac, there are also the amnion, the chorion and the allantois. In placental mammals besides all these membranes, the placenta is present too.
27. How is the yolk sac formed? What is the function of the yolk sac?
The yolk sac is formed from the covering of the vitellus by some cells originated from the primitive gut.
The yolk sac stores vitellus, the main nourishment source of nonplacental embryos.
Image Diversity: yolk sac
28. Which is the extra embryonic membrane whose function is to store nitrogen wastes of the embryo? Is this function present in placental mammalian embryos?
The allantois is the extra embryonic membrane whose function is to store excretes of the embryo.
In placental mammals, the allantois is present but it does not exert that function since the embryonic wastes are collected by the mother’s body through the placenta.
Image Diversity: allantois
29. Why can the allantois be considered an adaptation to terrestrial life?
The allantois is an adaptation to dry land because in embryos of oviparous terrestrial beings, like reptiles and birds, the metabolic residuals cannot be immediately excreted to the aquatic surrounds (as fishes and amphibian larvae do). It was necessary then the appearing of a structure capable of storing the embryonic excretes until hatching.
30. What is the difference between amnion and chorion?
Amnion is the membrane that covers the embryo. Chorion is the membrane that covers the amnion, the yolk sac, and the allantois. The space delimited by the chorion and the amnion is called amniotic cavity and it is filled with aminiotic fluid. The amniotic cavity has the functions of preventing desiccation of the embryo and of protecting it against mechanical shocks.
Image Diversity: amnion chorion
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Histogenesis is the process of tissue formation in the embryonic development. Organogenesis is the process of organ formation. Before histogenesis and organogenesis the primitive embryonic structures have been already formed: germ layers, neural tube, notochord, coeloms, somites.
22. What are twins? Genetically what are the two types of twins that can be generated?
Twins are simultaneously generated (within the mother’s uterus) offspring. Twins classify according to zygosity as monozygotic or as dizygotic twins.
Monozygotic twins, also known as identical twins, are those originated from one single fertilized ovum (therefore from one single zygote); monozygotic twins are genetically identical, i.e., they have identical genotypes and are necessarily of the same sex. Dizygotic twins, also known as fraternal twins, are those generated from two different ova fecundated by two different sperm cells; so they are not genetically identical and they are not necessarily of the same sex.
Image Diversity: twins
23. What is polyembryony?
Polyembryony is the phenomenon in which a single embryo in its initial embryonic stage divides itself forming many new individuals of the same sex and genetically identical. This is the way, for example, in which reproduction takes place in armadillos of the genus Dasypus. Polyembryony is an example of natural “cloning”.
24. What are extra embryonic membranes?
Extra embryonic membranes are membranous structures that appear paralleling the embryo and play important roles in the embryonic development. They form from the embryo but do not become part of the individual organism after its birth.
25. What are the extra embryonic membranes present in vertebrates?
The extra embryonic membranes that may be present in vertebrates are the yolk sac, the amnion, the chorion, the allantois and the placenta.
26. Are the extra embryonic membranes the same in all vertebrates?
The presence of each extra embryonic membrane varies according to the vertebrate class.
In fishes and amphibians, only the yolk sac is present. In reptiles and aves besides the yolk sac, there are also the amnion, the chorion and the allantois. In placental mammals besides all these membranes, the placenta is present too.
27. How is the yolk sac formed? What is the function of the yolk sac?
The yolk sac is formed from the covering of the vitellus by some cells originated from the primitive gut.
The yolk sac stores vitellus, the main nourishment source of nonplacental embryos.
Image Diversity: yolk sac
28. Which is the extra embryonic membrane whose function is to store nitrogen wastes of the embryo? Is this function present in placental mammalian embryos?
The allantois is the extra embryonic membrane whose function is to store excretes of the embryo.
In placental mammals, the allantois is present but it does not exert that function since the embryonic wastes are collected by the mother’s body through the placenta.
Image Diversity: allantois
29. Why can the allantois be considered an adaptation to terrestrial life?
The allantois is an adaptation to dry land because in embryos of oviparous terrestrial beings, like reptiles and birds, the metabolic residuals cannot be immediately excreted to the aquatic surrounds (as fishes and amphibian larvae do). It was necessary then the appearing of a structure capable of storing the embryonic excretes until hatching.
30. What is the difference between amnion and chorion?
Amnion is the membrane that covers the embryo. Chorion is the membrane that covers the amnion, the yolk sac, and the allantois. The space delimited by the chorion and the amnion is called amniotic cavity and it is filled with aminiotic fluid. The amniotic cavity has the functions of preventing desiccation of the embryo and of protecting it against mechanical shocks.
Image Diversity: amnion chorion
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