Online Lab Unit IV

The Photos above all show the different stages of development during the weeks of pregnacy.

About 1 month before conception: Almost all adult males produce thousands of spermatozoa (male germ cells) each second. It would take about 500 of them lined up in a row to total 1 inch in length. They take a month or so to travel from a testicle, through a long tube called the "vas deferens," to reach a small reservoir inside the man's prostate gland. Here, semen (a mixture of spermatozoa and various fluids) is formed. Each spermatozoon contains human DNA. They certainly appear to be living organisms. As seen in a microscope, they seem to be moving energetically with the sole motivation of fusing with an ovum. Most people consider them to be a form of human life, because they appear alive and contain human DNA. Some scientists define "life" so strictly that spermatozoon are not considered alive. Its movements are due to chemical reactions.
Perhaps one day before conception: The woman ovulates and produces one mature ovum (egg cell). It travels down one of her fallopian tubes towards her uterus. It is about 1/100" in diameter, and is barely visible to the naked eye. It also considered by most of the public to be a form of human life, for the above reasons. But it does not meet some scientists' strict definition of a living organism, because it lacks one factor: the ability by itself to reproduce. It can only reproduce with the assistance of a spermatozoon. Some of these scientists have described an ovum as an "inert globule of organic matter." It does carry a cargo of human DNA.
At conception: One very lucky spermatozoon out of hundreds of millions ejaculated by the man will penetrate the outside layer of the ovum and fertilize it. This happens typically in the outer third of one of the woman's Fallopian tubes. The surface of the ovum changes its electrical characteristics and normally prevents additional sperm from entering. A genetically unique entity is formed shortly thereafter, called a zygote. This is commonly referred to as a "fertilized ovum." However that term is not really valid because the ovum ceases to exist after conception. Half of the zygote's 46 chromosomes come from the egg's 23 chromosomes and the other half from the spermatozoon's 23. It has a unique DNA structure, different from that of the ovum and the spermatozoon. The zygote "...is biologically alive. It fulfills the four criteria needed to establish biological life:
metabolism,
growth,
reaction to stimuli, and
reproduction." 1
It can reproduce itself through twinning at any time up to about 14 days after conception; this is how identical twins are caused.Conception is the point that most, or all, pro-life groups and conservative Christians define as the beginning of pregnancy. 8 Most of these groups define the start of a human person as occurring at conception. The medical definition of the start of pregnancy is about 10 days later, at implantation. The zygote divides into two cells, called blastomeres. They subdivide once every 12 to 20 hours as the zygote slowly passes down the fallopian tubes.
About 3 days after conception: The zygote now consists of 16 cells and is called a 16 cell morula (a.k.a. pre-embryo). It has normally reached the junction of the fallopian tube and the uterus.
5 days or so after conception: A cavity appears in the center of the morula. The grouping of cells are now called a blastocyst. It has an inner group of cells which will become the fetus and later the newborn; it has an outer shell of cells which will "become the membranes that nourish and protect the inner group of cells." 3 It has traveled down the fallopian tubes and has started to attach itself to the endometrium, the inside wall of the uterus (a.k.a. womb). The cells in the inside of the blastocyst, called the embryoblast, start forming the embryo. The outer cells, called the trophoblast, start to form the placenta. It continues to be referred to as a pre-embryo. 2
9 or 10 days after conception: The blastocyst has fully attached itself to endometrium. Primitive placental blood circulation has begun. This blastocyst has become one of the lucky ones. Most never make it this far in the process.
12 days or so after conception: The blastocyst has started to produce hormones which can be detected in the woman's urine. This is is the event that all (or almost) all pro-choice groups and almost all physicians (who are not conservative Christians) define to be the start of pregnancy. If instructions are followed exactly, a home-pregnancy test may reliably detect pregnancy at this point, or shortly thereafter.
13 or 14 days after conception: A "primitive streak" appears. It will later develop into the fetus' central nervous system. This is the point at which spontaneous division of the blastocyst -- an event that sometimes generates identical twins -- is not longer possible. The pre-embryo is now referred to as an embryo. It is a very small blob of undifferentiated tissue at this stage of development.
3 weeks: The embryo is now about 1/12" long, the size of a pencil point. It most closely resembles a worm - long and thin and with a segmented end. Its heart begins to beat about 18 to 21 days after conception. Before this time, the woman might have noticed that her menstrual period is late; she might suspect that she is pregnant and conduct a pregnancy test. If it is an unwanted pregnancy, she might have already arranged and carried through with an abortion.
4 weeks: The embryo is now about 1/5" long. It looks something like a tadpole. The structure that will develop into a head is visible, as is a noticeable tail. The embryo has structures like the gills of a fish in the area that will later develop into a throat.
5 weeks: Tiny arm and leg buds have formed. Hands with webs between the fingers have formed at the end of the arm buds. Fingerprints are detectable. The face "has a distinctly reptilian aspect." 1 "...the embryo still has a tail and cannot be distinguished from pig, rabbit, elephant, or chick embryo." 3
6 weeks: The embryo is about 1/2" long. The face has two eyes on each side of its head; the front of the face has "connected slits where the mouth and nose eventually will be." 1
7 weeks: The embryo has almost lost its tail. "The face is mammalian but somewhat pig-like." 1 Pain sensors appear. Many conservative Christians believe that the embryo can feel pain. However, the higher functions of the brain have yet to develop, and the pathways to transfer pain signals from the pain sensors to the brain have not developed at this time.
2 months: The embryo's face resembles that of a primate but is not fully human in appearance. Some of the brain begins to form; this is the primitive "reptilian brain" that will function throughout life. The embryo will respond to prodding, although it has no consciousness at this stage of development. The brain's higher functions do not develop until much later in pregnancy.
10 weeks: The embryo is now called a fetus. Its face looks human; its gender may be detectable via ultrasound.
13 weeks or 3 months: The fetus is about 3 inches long and weighs about an ounce. Fingernails and bones can be seen. Over 90% of all abortions are performed before this stage. 9
17 weeks or 3.9 months: It is 8" long and weighs about a half pound. The fetus' movements may begin to be felt. Its heartbeat can usually be detected.
22 weeks or 5 months: 12" long and weighing about a pound, the fetus has hair on its head. Its movements can be felt. An abortion is usually unavailable at this gestational age because of state and province medical society regulations, except under very unusual circumstances. Half-way through the 22nd week, the fetus' lungs may be developed to the point where it would have a miniscule chance to live on its own. State laws and medical association regulations generally outlaw almost all abortions beyond 20 or 21 weeks gestation. "A baby born during the 22nd week has a 14.8 percent chance of survival. And about half of these survivors are brain-damaged, either by lack of oxygen (from poor initial respiration) or too much oxygen (from the ventilator). Neonatologists predict that no baby will ever be viable before the 22nd week, because before then the lungs are not fully formed." 4 Of course, if someone develops an artificial womb, then this limit could change suddenly.Fetal survival rate: "Most babies at 22 weeks are not resuscitated because survival without major disability is so rare. A baby's chances for survival increases 3-4% per day between 23 and 24 weeks of gestation and about 2-3% per day between 24 and 26 weeks of gestation. After 26 weeks the rate of survival increases at a much slower rate because survival is high already." 5
26 weeks or 6 months: The fetus 14" long and almost two pounds. The lungs' bronchioles develop. Interlinking of the brain's neurons begins. The higher functions of the fetal brain turn on for the first time. Some rudimentary brain waves can be detected. The fetus will be able to feel pain for the first time. It has become conscious of its surroundings. The fetus has become a sentient human life for the first time.
7 months: 16" long and weighing about three pounds. Regular brain waves are detectable which are similar to those in adults.
8 months: 18" long and weighing about 5 pounds.
9 months: 20" long and with an average weight of 7 pounds, a full-term fetus' is typically born about this time.

References that were used:
Carl Sagan, "Billions and Billions", Random House, New York NY (1997), Pages 163-179.
This ultrasound picture of a fetus at 2.8 month/12 week gestation was donated by a visitor to this web site.
C. George Boeree, "General Psychology: Prenatal development," at: http://www.ship.edu/~cgboeree/genpsyfetaldev.html
Franklin Foer, "Fetal Viability," The Gist, 1997-MAY-25, at: http://www.slate.com/id/1060/
"Chances for Survival," University of Wisconsin Medical School, 2004-APR-22, at: http://www.pediatrics.wisc.edu/patientcare/preemies/anticipating.html#survival