Biology - An Illustrated Guide to Science (794127), страница 12
Текст из файла (страница 12)
The egg passesdown the fallopian tu be toward theu teru s.● The corpus luteum produces ahormone called progesterone,which prevents development offurther ova.● If the egg isfertilized and thewoman becomesstromapregnant, thecorpus luteumlasts for up to fourmonths. If no pregnancyoccurs, it degenerates afterabout two weeks. Hormones from thepituitary cause the maturation ofanother follicle, and the whole processbegins again.●Maturation of Graafian follicle inside human ovary: schematicfollicle cellsuterusregressingcorpusluteumrupturedGraafianfollicleThe ovarian cycleFollicular phase: days 1–14.Primordial follicles develop into primaryand then secondary follicles; these secretethe hormone estrogen. Developmentproceeds through the tertiary follicle tothe Graafian follicle.OvulationThe Graafian follicle ruptures, releasingthe secondary oocyte.Luteal phase: days 14–28.The corpus luteum is formed from theruptured follicle.
It secretes the hormonesprogesterone and estrogen, finally shrinkingto become a scar.© Diagram Visual Information Ltd.theca (sheath)68CONTINUITYKey wordscervixsemenuterusSexual intercourseHuman sexual intercoursePreparing for coitusCoitus, or sexual intercourse,occurs when a man inserts his erectpenis into the vagina of a woman.The penis is kept erect by bloodthat floods into spongy tissue in thepenis at high pressure.● As sperm is released, it is mixedwith secretions from the seminalvesicles, prostate, and Cowper’sglands (see page 62) to producesem en .
Semen is a mixture ofsperm and a liquid containing sugarthat gives the sperm energy toswim.● At the same time, the walls of thevagina produce secretions that helpto lubricate the penis.fallopian tube●penisvas deferensuterusMaleIntercourseThe man inserts his penis into thewoman’s vagina. The head of thepenis reaches near the cervix,which is the base of the u teru sprojecting slightly into the vagina.● The male climax (the orgasm)occurs when semen containingsperm is forcefully ejected from thepenis into the vagina. The femaleclimax produces increasedsecretions from the vagina as wellas contractions of the uterus andvagina.●FemaleConception© Diagram Visual Information Ltd.●Conception occurs when aspermatozoon fuses with the egg.This must occur in the fallopiantubes.
Consequently, sperm mustswim from the vagina up throughthe uterus.ovaryvaginatestis69Human fertilizationCONTINUITYKey wordsFemale humancervixfertilizationmitosisspermzygoteSite of fertilizationDuring intercourse sperm is depositedin the vagina at the cervix, theentrance to the uterus.● Fertilization must occur high up inthe fallopian tube so that the ovumcan divide by m itosis before it attachesitself to the uterus.● The sperm must swim up through theuterus to reach this point.
Chemicalgradients guide the sperm towards theegg. This must occur within 72 hoursof ejaculation, or the sperm will benon-viable.●section throughovary, fallopian tube,and part of uterusSection through ovary, fallopian tube, and part of uterusEgg formation●first division by mitosisfallopian tubeeight-cell stage64-cell stagefertilizationembryo digestsits way intoendometriumspermpenetratesovumThe egg is released when the Graafianfollicle, the fluids-filled vesicle withinthe ovary containing the ovum,ruptures at the surface of the ovary.The funnel of the fallopian tube guidesthe egg into the tube where it starts itsjourney downward.FertilizationThe nucleus from one spermpenetrates the egg and fuses with theegg nucleus to form the first cell of thezygote.
This then divides repeatedly bymitosis to form a pair of cells and thenagain to form a ball of eight cells andso on. By the time a 64-cell ball hasbeen formed, it enters the uterus.● This ball of cells then has to embeditself in the endometrium, the wall ofthe uterus. Further development canthen occur.●ovulationfunnel offallopiantubeuterusrupturedfollicleovaryendometriumovumfluidfollicle cellsembryo embeddedin endometriumdevelopingovadevelopingfollicle© Diagram Visual Information Ltd.muscle layerGraafianfollicle70CONTINUITYKey wordscontraceptionembryofallopian tubegametevas deferensContraceptionCondom slipped ontoerect penisMale sterilizationTypes of contraceptionContraception covers all of thetechnologies that prevent a viableem bryo from forming or surviving.● There are three main types ofcontraception: barrier methods,sterilization, and the intrauterinedevice.vasdeferens●testisBarrier methodsBarrier methods prevent viable spermfrom meeting a viable egg.● A condom is a thin membrane of latexthat fits over the erect penis.
Thesperm cannot pass through thisbarrier, and the penis and condom areremoved from the vagina afterejaculation.● The diaphragm is a rubber cap that fitsover the cervix and prevents spermentering. The diaphragm must beleft in place after intercourse forsome time.●Diaphragm beinginserted into vaginaFemale sterilizationupper part offallopian tubelower partof fallopiantubeovaryuteruscervixSterilization●Sterilization prevents viablegam etes from meeting by cutting thetubes carrying them from the gonads(testes or ovary) to the oppositegender.
Male sterilization cuts the vasdeferen s. Female sterilization cuts thefallopian tu bes. Sterilization iseffectively permanent.vaginavaginarectumrectumDiaphragm in placeIntrauterine deviceIntrauterine devices(IUDs)© Diagram Visual Information Ltd.●Intrauterine devices, sometimescalled coils, prevent fertilized eggsfrom embedding in theendometrium. Once fitted, IUDsnormally remain in place for monthsor years.looputerusstringsvagina71TwinsCONTINUITYMonozygotic(identical) twinsKey wordsDizygotic( fraternal) twinsembryogenotypeovaryOvaryTypes of twinsThere are two types of twins: fraternalor dizygotic twins and identical ormonozygotic twins.● Other forms of multiple births(triplets, quads etc.) fall into the sametwo categories.●one ovum releasedtwo ova releasedIdentical twinsSometimes a fertilized ovum splits bymitosis into two cells, each of whichdevelops into a separate em bryo.● Since identical twins come from thesame egg and sperm, they aregenetically identical.● Identical twins are much rarer thanfraternal twins and are always the samegender.
Fertility treatments do notincrease the chances of identical twins.●FertilizationFertilized eggsplits into twoby mitosisFraternal twinsSometimes more than one egg isreleased from the ovaries at the sametime. If all of these eggs are fertilized,more than one embryo can be formed.● Since fraternal twins (and othermultiple births) develop from separateeggs and sperms, they have differentgen otypes (genetic combinations) andare only as similar as other brothers orsisters of the same age from the sameparents.● Fraternal twins can be differentgenders.
Some modern fertilitytreatments increase the rate ofmultiple egg production and so aremore likely to produce multiple births.In the uteruseach twinhas its ownplacentatwins sharea placentaTwinsidenticalIdentical twins share the samegenes, are of the same sex, andare often difficult to distinguish intheir appearance.fraternalFraternal twins are not necessarilyof the same sex, and are no morealike in their appearance or theirgenes than ordinary siblings.© Diagram Visual Information Ltd.●Each celldevelopsinto a newindividual72CONTINUITYKey wordsamnionfetusplacentaFetal developmentFetus at 6 weeks0 .5 inches (12 mm)Uterus at 6 weeksfallopiantubefetusplacentaThe first trimesteruterusThe first trimester covers the firstthree months after conception. Mostof the key structures are laid downduring this period.● Teratogenic chemicals (chemicals thatproduce birth deformities) are mostdangerous during this time.● By the end of the first trimester thefetu s can be recognized as male orfemale.
It has a well-developedplacen ta linking it to the mother. Thefluid-filled sac called the am n ion hasformed and surrounds the growingfetus with a bag of waters to protect itfrom mechanical damage. The fetuswill be about 3 inches (80 mm) longby the end of this stage.cervix●vaginaUterus at 10 weeksamniotic fluidamnionThe second trimester●© Diagram Visual Information Ltd.Uterus at 20 weeksDevelopment continues during thesecond trimester, and by 20 weeks thefetus is able to produce digestiveenzymes and move itself. Mothers willfeel kicks from the baby by this stage.The third trimesterDuring the third trimester the fetuscontinues to grow in size. Most of thekey body parts have developed by thisstage, and babies born duringthis time can normallysurvive outside themother, although they willbe small and will needspecial care.● By the end of the thirdtrimester, the baby is ready to be born.It will now weigh approximately 7 or 8pounds (3–4 kg).
The placenta willweigh almost as much.●umbilicalcordFetus at 10 weeks2.4 inches (60 mm)Fetus at 20 weeks9.8 inches (250 mm)Full-term fetus73PlacentaCONTINUITYKey wordsFetus in the uterusamnionfetusplacentaumbilical corduterusamniotic fluiduterine wallfetusdetail ofplacentaSource of the placenta●placentaembryonic membranesvillusThe placen ta is an organ thatdevelops from cells from the fetu s andthe wall of the u teru s. Despite this,the blood supplies and cells of fetusand mother are kept separate,although chemicals can pass easilyacross the barrier.Function of the placentaumbilical cordThe placenta supplies the growingfetus with oxygen and food, andremoves waste products such ascarbon dioxide.● The placenta allows this exchange butalso keeps the fetus and motherseparate—they are separateindividuals.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
