At the end of the first week, the blastocyst comes in contact with the uterine wall and adheres to it, embedding itself in the uterine lining via the trophoblast cells. Thus begins the process of implantation, which signals the end of the pre-embryonic stage of development ([link]).
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During prenatal development, the fetal circulatory system is integrated with the placenta via the umbilical cord so that the fetus receives both oxygen and nutrients from the placenta. However, after childbirth, the umbilical cord is severed, and the newborn’s circulatory system must be reconfigured.
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The embryonic period in humans begins at fertilization (penetration of the egg by the sperm) and continues until the end of the 10th week of gestation (8th week by embryonic age). The period of two weeks from fertilization is also referred to as the germinal stage.
https://en.wikipedia.org/wiki/Prenatal_development
By the end of this section , you will be able to : * Distinguish the stages of embryonic development that occur before implantation * Describe the process of implantation * List and describe four embryonic membranes * Explain gastrulation * Describe how the placenta is formed and identify its functions * Explain how an embryo transforms from a flat ...
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A developing human is referred to as an embryo during weeks 3–8, and a fetus from the ninth week of gestation until birth. In this section, we’ll cover the pre-embryonic and embryonic stages of development, which are characterized by cell division, migration, and differentiation.
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The placenta develops throughout the embryonic period and during the first several weeks of the fetal period; placentation is complete by weeks 14–16. As a fully developed organ, the placenta provides nutrition and excretion, respiration, and endocrine function ([link] and [link]).
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The eyes are well-developed by this stage, but the eyelids are fused shut. The fingers and toes begin to develop nails. By the end of week 12, the fetus measures approximately 9 cm (3.5 in) from crown to rump. Weeks 13–16 are marked by sensory organ development.
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After fertilization, the process of embryogenesis (the early stages of prenatal development) begins. By the end of the tenth week of gestational age the embryo has acquired its basic form and the next period is that of fetal development where the organs become fully developed. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age.
https://en.wikipedia.org/wiki/Prenatal_development
Prenatal or antenatal development is the process in which a human embryo and later fetus (or foetus) develops during pregnancy, from fertilization until birth. Often, the terms fetal development, or embryology are used in a similar sense.
https://en.wikipedia.org/wiki/Prenatal_development
Throughout this chapter, we will express embryonic and fetal ages in terms of weeks from fertilization, commonly called conception. The period of time required for full development of a fetus in utero is referred to as gestation (gestare = “to carry” or “to bear”).
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what is the first organ to develop in the embryonic state
The placenta is the first organ to develop in the embryonic state. Placenta is the first organ to develop in the embryonic state.