The time table of human development, taken from a standard human embryology text (often used in medical school anatomy classes), Langman, Medical Embryology, Williams and Wilkins Press:
While some of this is technical, there is still information of value here.
*day 1 fertilization process - sperm enters egg, conception occurs.
*day 2 two cell stage (30 hours), 4 cell stage (40 hours)- if a cell falls off the cluster, it can develop into another embryo plus placenta, i.e, identical twin)
|This image was borrowed without permission|
from the Advanced Fertility Center of Chicago
who specialize in in-vitro fertilization for those
who are having difficulty conceiving
The developing fetus itself is the area marked
as "ICM" (inner cell mass)
The blastocyst cavity in the center is marked
The trophectoderm cells that will form the
placenta surround the cavity - one is marked
with a "T"
*day 3 morula (solid ball of cells)
*day 4 early blastocyst (ball of cell forms a small central cavity - cells are now committed to becoming either placental cells or embryo cells, though they can't be recognized as such)
*day 6-7 implantation starts. Group of cells at one pole of the cyst lining is now recognizable as the embryonic precursor tissue.
*day 8 two-layer disc of embryonic tissue
*days 8-12 conceptus gradually covered by maternal tissue, maternal placenta bed vessels expand, placental primitive vessels start to form
*day 13 uteroplacental circulation begins
*day 14 embryonic disc forms "primitive streak," from which a third layer of embryonic cells emerge
Menstruation is now due.
*day 15 three-layer embryonic disc complete
*day 17 notochord forms (axial scaffold later replaced by spinal column)
*day 19 top layer of disc (ectoderm) forms a central flat strip (neural plate) that is the nervous system precursor
*day 20 middle layer of disc (mesoderm) starts partitioning into segments called somites, which will later become ribs, muscles, etc.
*days 20-23 neural plate rolls into a tube and sinks below the surface of the ectoderm. This tube will form the spinal cord and brain eventually.
Menstruation now 1 week late
*days 21-22 heart tube forms (yes, a simple tube). Other vessels and blood cells also form at this time.
Menstruation now 2 weeks late - 28 days from conception
*days 28-31 arm, leg, and tail buds form. These look like flippers. Endoderm layer gives rise to lung, liver buds.
Menstruation now 3 weeks late
*week 5 kidney buds form. Eye and lens buds form in association. Heart tube folds and forms the four chambers. Heart now beating.
Menstruation now 4 weeks late
*week 6 "indifferent" gonads form, having no male or female characteristics at this time. External genitalia also indistinguishable. Hand and foot plates start to separate the fingers and toes.
Menstruation now 6 weeks late
*week 8 lung buds continue to branch. Gonads now distinguishable as ovary or testis. Eyes, nostrils, mouth identifiable externally. Primitive uterus and fallopian tubes form.
*** By the end of the 8th week, all major organs have their basic forms. The rest of development is required to make the organs functional. Most major anatomic birth defects are caused by damage during the 3rd to 8th week of gestation.
This is why it is so important, when using herbs to terminate pregnancy, if they should fail, to follow up with a clinical abortion. As you can see, in the first few weeks of pregnancy are vital to the survival of the developing child, if any of these processes are disrupted, as by herbs or other substances, the risk of birth defects is pronounced, from these few cells comes all the groundwork for the future person, the foundation for the major organs, brain and spinal column are laid at this time.
The site where I obtained the above image, Advanced Fertility Center of Chicago has a lot of information and images about the very first days of pregnancy. They specialize in in-vitro fertilization (in the test tube) for couples who are having difficulty conceiving, and have many pictures of what the developing egg looks like before it is implanted in the uterus and is worth checking out if you are interested.