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.
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:
Basing on the the accepted model of external fertilization, basic biochemical procedures involved can be stated as
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.
Fig 1: Salmon life cycle, showing external fertilizatioon Fig 2: Salmon eggs,"eyed" ones fertilized
Fig 3: Fry of carps Fig 4: Mating in frogs