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Implantation &The Embryonic Period of Development

Implantation refers to the attachment of the zygote to the wall of the uterus . The zygote generally implants at the top of the uterus, near where it exits the fallopian tube , beginning between 5 and 8 days after fertilisation and is completed by 9 or 10 days post- conception . The outer layer of this cell mass or trophoblast , attaches itself by secreting proteolytic enzymes, which erode the uterine wall cells, gradually embedding itself firmly in the uterine wall. However, in certain cases the fertilised egg may develop outside the uterus (cf. ectopic pregnancy ).

We have now entered the embryonic period of development (2 to 8 weeks post-conception), where the zygote is now referred to as the embryo .

As the zygote implants and becomes the embryo, the blastocyst begins to form 2 layers:

  • The inner layer of cells is called the endoderm and eventually develops into the digestive and respiratory systems.
  • The outer layer is divided into 2 parts: the ectoderm (outermost layer of cells) and mesoderm (middle layer; between the endoderm and ectoderm). The ectoderm will develop into the nervous system, sensory receptors (eyes, ears, nose, etc.) and skin (including nails and hair), while the mesoderm will become the bones, muscles, excretory, circulatory and reproductive systems.

While these layers of the embryo form, so too do the life-support systems for the developing embryo, from the outer layer of the cell mass.

The next stage of development, from month 2 until birth , is the foetal period of development .

The information in this page is presented in summarised form and has been taken from the following source(s):
1. Child Development , 6th Edition (1994), J. W. Santrock, Wm. C. Brown Communications, Inc.
2. Introduction to Human Physiology , 2nd Edition (1981), M. Griffins, Macmillan Publishing Co. Inc, New York

Other HON resources 
   From MedHunt

Embryonic Development
    From HONselect
     (def;articles & more)   

Ovum Implantation
Growth and Embryonic Development:

Umbilical Cord
Fetal Blood:

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Ovum Implantation
Growth and Embryonic Development
Umbilical Cord
Fetal Blood


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Contact Last modified: Jun 25 2002