REPRODUCTION IN ANIMALS D

1 REPRODUCTION IN ANIMALSREPRODUCTION IN ANIMALSDo you recall the processes ofdigestion, circulation andrespiration which you havestudied in your previous classes? Theseprocesses are essential for the survivalof every individual. You have also learntabout the process of REPRODUCTION inplants. REPRODUCTION is essential for thecontinuation of a species. Imagine whatwould have happened if organisms hadnot reproduced. You will realise thatreproduction is very important as itensures the continuation of similarkinds of individuals, generation have already learnt in yourprevious class about REPRODUCTION inplants. In this chapter, we shall learnhow REPRODUCTION takes place Modes of ReproductionHave you seen the young onesof different ANIMALS ?

2 Try to name someof the young ones by completingTable shown in examples at S. and must have seen the youngones of various ANIMALS being you tell how chicks andcaterpillars are born? How are kittensand puppies born? Do you think thatthese young ones looked the samebefore they were born as they do now?Let us find as in plants, there are two modesby which ANIMALS reproduce. These are:(i) Sexual REPRODUCTION , and(ii)Asexual Sexual ReproductionTry to recall REPRODUCTION in plantswhich you studied in Class VII. You willremember that plants that reproducesexually have male and femalereproductive parts. Can you name theseparts? In ANIMALS also, males andfemales have different reproductive partsor organs. Like plants, the reproductiveparts in ANIMALS also produce gametesthat fuse to form a zygote.

3 It is the zygotewhich develops into a new type of REPRODUCTION beginningfrom the fusion of male and femalegametes is called sexual us find out the reproductive partsTable 22in humans and study the process ofreproduction in Reproductive OrgansThe male reproductive organs include apair of testes (singular, testis), two spermducts and a penis (Fig. ). The testesproduce the male gametes calledsperms. Millions of sperms are producedby the testes. Look at Fig. whichshows the picture of a sperm. Thoughsperms are very small in size, each hasa head, a middle piece and a tail. Doesit appear to be a single cell? Indeed, eachsperm is a single cell with all the usualcell :Male reproductiveorgans in humansWhat purpose does thetail in a sperm serve?

4 Fig. :Female reproductive organs inhumansFemale Reproductive OrgansThe female reproductive organs are a pairof ovaries, oviducts (fallopian tubes) andthe uterus (Fig. ). The ovary producesOviductOvaryUterusHeadMiddle pieceTailFig. : Human spermREPRODUCTION IN ANIMALS101 SpermductTestisPenis2021 22 SCIENCE102 Boojho recalls that the sizeof eggs in ANIMALS egg may be very smallas in humans, much largeras in hens. Ostrich egg isthe largest!FertilisationThe first step in the process ofreproduction is the fusion of a spermand an ovum. When sperms come incontact with an egg, one of the spermsmay fuse with the egg. Such fusion ofthe egg and the sperm is calledfertilisation (Fig. ). Duringfertilisation, the nuclei of the sperm andthe egg fuse to form a single results in the formation of afertilised egg or zygote (Fig.)

5 DidFig. : FertilisationFig. : Human OvumNucleusSpermsOvumFusingnucleiThe process of fertilisation is themeeting of an egg cell from the motherand a sperm cell from the father. So,the new individual inherits somecharacteristics from the mother andsome from the father. Look at yourbrother or sister. See if you can recognisesome characters in them similar to thoseof your mother or your which takes place insidethe female body is called internalfertilisation. Internal fertilisationoccurs in many ANIMALS includinghumans, cows, dogs and : Zygotefemale gametes called ova (eggs)(Fig. ). In human beings, a singlematured egg is released into the oviductby one of the ovaries every is the part where development ofthe baby takes place.

6 Like the sperm,an egg is also a single know that the zygote is thebeginning of a new individual?2021 22 REPRODUCTION IN ANIMALS103 Why do fish andfrogs lay eggs inhundreds whereas ahen lays only oneegg at a time?Have you heard oftest tube babies?Boojho and Paheli s teacher once toldthem in the class that in some womenoviducts are blocked. These womenare unable to bear babies becausesperms cannot reach the egg forfertilisation. In such cases, doctorscollect freshly released egg andsperms and keep them together for afew hours for IVF or in vitrofertilisation ( fertilisation outside thebody). In case fertilisation occurs,the zygote is allowed to develop forabout a week and then it is placed inthe mother s uterus.

7 Completedevelopment takes place in the uterusand the baby is born like any otherbaby. Babies born through thistechnique are called test-tubebabies. This term is actuallymisleading because babies cannotgrow in test : Eggs of frogfemale lays hundreds of eggs. Unlikehen s egg, frog s egg is not covered by ashell and it is comparatively verydelicate. A layer of jelly holds the eggstogether and provides protection to theeggs (Fig. ).You will be surprised to know thatin many ANIMALS fertilisation takesplace outside the body of the these ANIMALS , fertilisation takesplace in water. Let us find out howthis some ponds or slow-flowingstreams during spring or rainyseason. Look out for clusters offrog s eggs floating in down the colour and size ofthe spring or rainy season, frogsand toads move to ponds and slow-flowing streams.

8 When the male andfemale come together in water, theAs the eggs are laid, the maledeposits sperms over them. Eachsperm swims randomly in water withthe help of its long tail. The spermscome in contact with the eggs. Thisresults in fertilisation . This type offertilisation in which the fusion of amale and a female gamete takes placeoutside the body of the female is calledexternal fertilisation . It is verycommon in aquatic ANIMALS such asfish, starfish, 22 SCIENCE104 Development of EmbryoFertilisation results in the formation ofzygote which begins to develop into anembryo [Fig. (a)]. The zygote dividesrepeatedly to give rise to a ball of cells[Fig. (b)]. The cells then begin to formgroups that develop into different tissuesand organs of the body.

9 This developingstructure is termed an embryo. Theembryo gets embedded in the wall of theuterus for further development[Fig. (c)].The embryo continues to develop inthe uterus. It gradually develops bodyFig. :(a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells(enlarged); (c) Embedding of the embryo in the uterus (enlarged)How could a single cellbecome such a bigindividual?(b)(a)(c)UterusEmbeddingem bryoOvaryOvulationZygoteUteruswallDevelo pingembryoThough these ANIMALS layhundreds of eggs and releasemillions of sperms, all the eggsdo not get fertilised and developinto new is because the eggs andsperms get exposed to watermovement, wind and , there are other ANIMALS inthe pond which may feedon eggs.

10 Thus, productionof large number of eggs andsperms is necessary toensure fertilisationof at leasta few of 22 REPRODUCTION IN ANIMALS105parts such as hands, legs, head, eyes,ears etc. The stage of the embryo inwhich all the body parts can beidentified is called a foetus (Fig. ).When the development of the foetus iscomplete, the mother gives birth tothe inside the egg shell during thisperiod? After the chick is completelydeveloped it bursts open the egg ANIMALS which undergo externalfertilisation, development of the embryotakes place outside the female body. Theembryos continue to grow within theiregg coverings. After the embryos develop,the eggs hatch. You must have seennumerous tadpoles swimming in pondsand and Oviparous AnimalsWe have learnt that some ANIMALS givebirth to young ones while some animalslay eggs which later develop into youngones.