1.External fertilization and External development:
This pattern is found in many aquatic animals,such as Obelia, Nereis,all bony fishes and frogs.Parents release sperms and eggs into the surrounding water,where fertilization occurs and zygote develop into offspring.
2.Internal fertilization and External development:
Sperms are passed from the male into the female with an intromittent organ,such as claspers in male shark,hemi penis in lizard,snake,crocodile and penis in mammals,or otherwise,for example, by cloacal apposition in birds,with modified arm in cuttle fish.Internal fertilization has several advantages.
Animals with internal fertilization usually produce fewer zygotes because of protection provided by egg shells or internal development.
3.Internal fertilization and Internal development.
Internal development provides additional advantages to the embryo. The mother's body provides exactly the right chemical conditions and in mammals, warmth and nourishment also.As the mother carries the embryo wherever she goes,it is not vulnerable to predators who attack externally develpment eggs.
Characteristics of sexual reproduction:
Sexual reproduction has the following important basic features:-
Significance of sexual reproduction:
Sexual reproduction has a dual significance for the species:
Development of the offspring from reproductive units,such as buds or fragments, in asexual reproduction is called blastogenesis.Development of the embryo from the zygote in sexual reproduction is termed embryogenesis.
Unisexual or dioecious:
Organism in which the two sexes occur in different individuals. e.g.,Humans,mammals, birds,lizards.
Organism in which the two types of sex organs(testes and ovaries) occur in the same individual,e.g.,Earthworm, Taenia,Leech,Fasciola etc.
Human reproductive sytem:
In human beings,reproduction takes place by sexual method and sexes are separate.
(i)Primary sex organs:
Gonads which form gametes are called primary sex organs-testis(plural testes) in males and ovary(plural ovaries) in females.Testis produces sperms and secretes testosterone. Ovary produces ova.Maturing Graffian follicles secrete oestrogens.
(ii)Secondary sex organs:-
Sex organs,glands and ducts which do not produce gametes but are otherwise essential for sexual reproduction are known as secondary sex organs.In human male reproductive system,the secondary sex organs are vasa efferentia,epididymis,vasa deferentia,ejaculatory ducts,urethra,accessory sex glands are prostate glands,Cowper's glands seminal vesicle and penis. Secondary sex organs of a human female include fallopian tubes,uterus,vagina, external genitalia,Bartholin's gland and mammary glands are accessory sex glands.
(iii)Accessory/External/ Secondary sex characters:
They are traits which do not have any direct role in reproduction but provide specific features and structures to the two sexes.The important external/accessory sex characters of human male are beard,moustaches, body hair on shoulder and chest,pubic hair on both lateral and vertical directions, comparatively more height with more muscular body,larynx apparent externally, voice low pitched with breathing more by means of diaphragm.The important accessory sex character of human female are high pitched voice,breast,border pelvis,lateral pubic hair,rounded body contours with more subcutaneous fat in thighs,buttocks and face and sternal breathing.
What is puberty:
Beginning of sexual maturity or ability to reproduce is known as Puberty. Puberty occurs at the age of 10-14 years in girls and 13-15 years in boys.
Characteristics of human reproduction: