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External Fertilization

Introduction to external fertilization :

Fertilization is the fundamental process of reproduction in sexually reproducing  animals.in this process , diploid (n) gametes of the male and the female parents unite together  to form a diploid zygote ,and the process is called  fertilization.  Fertilization in itself is a complicated process involving interactions of several factors ,chemical substances, etc that  orchestrate the process of union between the sperms and the ova.Fertilization can occur inside the body of the mother, (internal fertilization like man) or outside the mother's body in water  -called external fertilization  like in fishes, frogs etc.  Internal fertilization developed in evolution when the animals started to migrate onto the land from water. Initially when life was only in water, there was no problem of leaving eggs without the fear of them getting dried out . But as the species tried out on land, that condition was altered.This was solved by development of an egg that was provided with its water  and food for the embryo,called a cleidoic egg (like the hen's egg).  this development was ushered in the reptiles which are oviparous . With time , internal fertilization developed but the aspect of external fertilization still remains the mainstay for aquatic,sexually reproducing, animals.

External Fertilization

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The process of external  fertilization is a physio -biochemical process by means of which the haploid sperms and ova  unite together to form a diploid zygote.

The main feature of external fertilization are:

  • Both the  male and female parents interact only during a specific breeding period and spawning is observed
  • In some animals like fishes or certain frogs. the process  differs a little where parental care is observed as far as care of the young are concerned
  • The eggs and the sperm are released together after a little sport ,under appropriate conditions of temperature, light, rainfall etc
  • The sperms find their way to the ova by  chemo-taxis involving specific chemicals
  • The fertilized embryo develops inside the  egg covered by thin shell, in certain cases, they may  be carried by the mouth, or borne on the back
  • The developed embryo hatches into a larva  which undergoes further development  into  fingerling, fry ,then into adult
  • The matured larva can undergo progressive metamorphosis as the tadpole larva develops into a frog

External Fertilization : Cellular Interaction in External Fertilization

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Basing on the the accepted model of external fertilization, basic biochemical procedures  involved  can be stated as

  1. Approach of spermatozoa towards the egg
  2. Contact and recognition]
  3. Acrosome reaction
  1. Prevention to polyspermy
  2. Amphimixix or zygote formation


The spermatozoa approaches the egg drawn by the increasing concentration of some peptides like speract, resact etc. These bind with the trans membrane receptors of the sperm membrane and draw it in favor of the concentration gradient. The contact, recognition and the acrosome reaction are all part of the primary stages of (external) fertilization in which the acrosome derived from the Golgi complex releases sperm lysins (enzymes) that perforate the egg membrane and makes the sperm path for the nucleus of the sperm. In later stages, after entry of the nucleus and the proximal centriole from the sperm, the formation of spindle fibers occurs from the centriole while the nuclei change into male and female pronucleus by hydration.The spindle fibres pull the two pro nuclei together  to form the zygote which immediately stars to divide by cleavage.

External Fertilization : Advantages

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  • External fertilization does not require specialized structures like placenta etc for survival of foetus
  • It is prevented from dessication by environment
  • There is a potential for high rate of reproduction
  • Parental care is not essential
  • Developing larvae are supplied by the egg resources itself(yolk)
  • Hatched Young's are independent and can find food for themselves

External Fertilization : Images

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Life cycle of Salmon showing   External fertilization              Fry of common carps                

Fig 1: Salmon life cycle, showing external fertilizatioon                    Fig 2: Salmon eggs,"eyed" ones fertilized


   Eggs of Salmon fertilized ones  with "eyes"                Mating of frog after  which eggs are liberated 
                           Fig 3: Fry of carps                                                                            Fig 4: Mating in frogs          


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