Tip:
Highlight text to annotate it
X
BETWEEN THE 8th AND 9th WEEK OF DEVELOPMENT,
THE DIFFERENTIATION OF CELLS IN THE EMBRYO HAS BEEN ESTABLISHED.
ALL HUMAN FEATURES ARE EVIDENT,
AND THE EMBRYO IS NOW CALLED THE FETUS.
DO EITHER OF YOU KNOW
WHAT DISTINGUISHES BETWEEN AN EMBRYO AND A FETUS?
WELL, WHEN THE MASS OF CELLS CHANGE TO SPECIALIZED TISSUE,
THEN WE CALL IT A FETUS.
IT ALSO STARTS TO TAKE ON THE FORM OF A HUMAN, TOO,
AS YOU CAN SEE HERE.
AT WHAT TIME DURING GESTATION WOULD THAT OCCUR?
GESTATION IN HUMANS IS 266 DAYS
OR 280 DAYS SINCE THE LAST MENSTRUAL FLOW.
SO, AN EMBRYO WOULD BECOME A FETUS
WITHIN THE FIRST 2 MONTHS OR SO OF DEVELOPMENT, RIGHT?
YES. GESTATION IS CONVENIENTLY DIVIDED INTO 3 TRIMESTERS,
EACH ABOUT 3 MONTHS LONG.
THE EMBRYO BECOMES THE FETUS
DURING THE FIRST 8 TO 9 WEEKS OF THE FIRST TRIMESTER.
9 WEEKS.
IS THIS THE AVERAGE SIZE OF A 9-WEEK-OLD FETUS?
MM-HMM.
LOOKS TO BE ABOUT 4 OR 5 CENTIMETERS LONG.
I THINK THAT'S PROBABLY A PRETTY GOOD ESTIMATE, CAM.
WHY DON'T YOU MEASURE IT?
TAKE THE MEASUREMENT FROM THE TOP OF ITS HEAD TO ITS BUTTOCKS.
THAT'S CALLED THE CROWN-RUMP LENGTH.
AND WHAT IS IT?
IT'S ABOUT 5 CENTIMETERS.
HMM.
WHAT STARTED AS AN EMBRYO HAS NOW BECOME A FETUS,
OR A DEVELOPING BABY.
IS IT POSSIBLE TO TELL
WHETHER IT'S A MALE OR A FEMALE AT THIS STAGE?
*** DIFFERENCES DO EXIST IN THE EXTERNAL GENITALIA,
BUT THEY'RE NOT OBVIOUS TO THE AVERAGE PERSON --
ONLY TO SPECIALISTS.
HOW DOES THE EMBRYO KNOW
TO DEVELOP MALE OR FEMALE GENITALIA?
WELL, THAT'S DETERMINED BY ITS GENETIC MAKEUP.
QUITE SIMPLY, MOTHERS AND FATHERS HAVE SEX CHROMOSOMES --
XX IN THE MOTHER, XY IN THE FATHER.
ALL EGG CELLS CONTAIN AN "X" CHROMOSOME.
AND *** CAN CARRY EITHER "X" CHROMOSOME OR "Y" CHROMOSOME.
AT FERTILIZATION, THE EGG CARRYING THE "X" CHROMOSOME
MAY BE FERTILIZED BY A *** CELL CARRYING AN "X."
IF THAT'S THE CASE,
THE EMBRYO IS XX AND WILL DEVELOP INTO A BABY GIRL.
IF THE EGG IS FERTILIZED
BY A *** CELL CARRYING A "Y" CHROMOSOME,
THEN A BABY BOY WILL DEVELOP BECAUSE THE EMBRYO IS XY.
SINCE THE FATHER IS CONTRIBUTING THE DIFFERENT SEX CHROMOSOME,
IS IT FAIR TO SAY
THAT HE IS THE ONE THAT DETERMINES THE SEX OF THE CHILD?
THAT'S WHAT EMBRYOLOGISTS BELIEVE,
BUT ALL SUBSEQUENT EVENTS
THAT DETERMINE THE *** DIFFERENCES
BETWEEN MALES AND FEMALES
DEPEND ON WHICH *** DEVELOP
AND WHAT HORMONES THEY PRODUCE.
IT'S THE ANDROGENS
THAT INFLUENCE THE DEVELOPMENT OF THE ***.
OH, GOOD MEMORY, SANDRA.
IF ANDROGENS, OR MALE SEX HORMONES,
ARE PRODUCED BY THE DEVELOPING EMBRYO,
THEN MALE SEX ORGANS WILL BEGIN TO FORM.
BUT IF NO ANDROGENS ARE PRESENT,
THEN FEMALE *** WILL DEVELOP.
*** OF BOTH MALE AND FEMALE FETUSES BEGIN THEIR DEVELOPMENT
DURING THE FIFTH WEEK OF GESTATION.
THEY START FROM A MASS
OF EMBRYONIC TISSUE CALLED THE MESODERM.
AT FIRST, THE *** ARE INDISTINGUISHABLE
AS MALE OR FEMALE.
THEY SIMPLY FORM A GONADAL RIDGE.
WHEN STEM CELLS,
OR PRECURSOR REPRODUCTIVE CELLS
DESTINED TO BECOME SPERMATOGONIA AND OOGONIA
MIGRATE FROM THE MESODERM TO THE RIDGE,
DIFFERENTIATION INTO THE *** OR OVARIES IS TRIGGERED.
IN MALES, THE SEMINIFEROUS TUBULES AND ASSOCIATED DUCTS
DEVELOP AT 7 WEEKS.
IN FEMALES, IMMATURE OVARIES AND DUCTS
DEVELOP AT THE 8-WEEK STAGE.
EXTERNAL GENITALIA ALSO DEVELOP RAPIDLY DURING WEEK 8.
DIFFERENTIATION OF THE EXTERNAL GENITALIA
AND ACCESSORY STRUCTURES
ARE TOTALLY DEPENDENT
ON THE PRESENCE OR ABSENCE OF TESTOSTERONE,
WHICH IS PRODUCED BY THE ***.
THIS IS A MODEL OF A 3-MONTH-OLD FETUS.
BY THE END OF THE FIRST TRIMESTER,
THE FETUS IS LOOKING VERY MUCH MORE HUMAN.
IT ALSO STARTS TO MOVE AROUND --
MAY ACCIDENTALLY KICK OR POKE ITS MOTHER
AS IT EXPLORES ITS AMNIOTIC ENVIRONMENT.
DOES THE MOTHER FEEL THIS? OH, SURE.
ALSO, THE STARTLE REFLEX AND THE SUCKING ACTION FIRST APPEAR.
FACIAL EXPRESSIONS ARE QUITE COMMON.
AND NOW IT CAN BE CLEARLY IDENTIFIED
AS A MALE OR A FEMALE.
BUT IT'S STILL SO SMALL.
IT'S PROBABLY ONLY ABOUT 10 CENTIMETERS LONG
AND PROBABLY ONLY WEIGHS A COUPLE OF GRAMS.
WELL, ACTUALLY, IT PROBABLY WEIGHS ABOUT 15 GRAMS.
BUT ITS URINARY SYSTEM AND ITS RESPIRATORY SYSTEM
ARE QUITE WELL DEVELOPED NOW.
BUT IT STILL RECEIVES OXYGEN
AND TRANSFERS WASTE THROUGH THE PLACENTA, RIGHT?
OH, OF COURSE.
NOW, THE 4-TO-6-MONTH STAGE -- DO YOU NOTICE ANY CHANGES?
WELL, THE PLACENTA SEEMS TO BE
TAKING UP QUITE A BIT MORE OF THE WOMB,
AND THE FETUS IS A LOT LARGER, TOO.
AND WE CAN'T SEE IT ON THIS MODEL,
BUT WE SHOULD SEE SOME HAIR ON THE BODY AND ON THE HEAD.
RIGHT. BY THE END OF THE SECOND TRIMESTER,
THE CROWN-TO-RUMP LENGTH OF A FETUS
SHOULD BE ABOUT 28 CENTIMETERS.
IT WOULD WEIGH BETWEEN 680 AND 700 GRAMS.
WE WOULD BE ABLE TO DETECT ITS SKELETON
USING X-RAY TECHNOLOGY --
COULD EVEN HEAR ITS HEARTBEAT WITH A STETHOSCOPE.
HOW FAST DO YOU THINK THAT HEART WOULD BE BEATING?
WELL, IT'S MUCH FASTER THAN OUR AVERAGE HEART RATE OF 70,
SO, AROUND 100 BEATS PER MINUTE?
EVEN HIGHER THAN THAT, SANDRA.
A BABY'S HEARTBEAT CAN BE 120 TO 160 BEATS PER MINUTE.
NOW, IN THE SEVENTH-MONTH STAGE,
DO YOU NOTICE ANY VISIBLE CHANGES?
WELL, IT'S MORE THAN DOUBLED IN SIZE.
YES. THIS IS A PERIOD OF VERY ACTIVE GROWTH.
BY THE END OF THE NINTH MONTH,
THE BABY WILL WEIGH 3,400 GRAMS, OR ABOUT 7 1/2 POUNDS,
AND BE 35 TO 40 CENTIMETERS IN LENGTH.
SEVERAL NEURAL TRACTS HAVE FORMED,
AND THE NUMBER OF BRAIN CELLS IS INCREASING RAPIDLY.
IN 9 MONTHS, A FETUS SURE GOES THROUGH A LOT OF CHANGES.
YOU'RE ABSOLUTELY RIGHT.
DO YOU KNOW THAT IT'S BEEN ESTIMATED
THAT IF A BABY WERE TO CONTINUE THE SAME GROWTH RATE
AS IT HAS IN FETAL DEVELOPMENT
THAT BY ITS FIRST BIRTHDAY,
IT MIGHT WEIGH 90 KILOGRAMS, OR 200 POUNDS?