Comparative Anatomy - ZOO 420 2(1+1) Zoology Department Dr. Elsayed Younis

Reproduction, Fertilization and Cleavage Reproduction: Is the biological process by which new individual organisms "offspring" are produced from their "parents". Reproduction: is a fundamental feature of all known life; each individual organism exists as the result of reproduction. Types of Reproduction: There are two forms of reproduction: Asexual and Sexual Asexual reproduction: an organism create identical copy of himself without the contribution of another organism. Sexual reproduction: requires the sexual interaction of two specialized organisms( produces male gamete which fertilizes the female gamete to create fertilizing egg " Zygote"

Fertilization: is the fusion of gametes to initiate the development of a new individual organism. Types of Fertilization: A- Allogamy : is the fertilization (combination) of gametes from two parents B- Autogamy (Self- fertilization) : occurs in hermaphroditic organisms where the two gametes come from the same individual C- Internal fertilization: The male deposits his sperm directly into the female's body. D- External fertilization: The male and female's gametes unite outside the female's body.

Cleavage: The zygotes undergo rapid cell division producing a cluster of cells with the same size as the original zygote. Cleavage ends with the formation of the blastula. Types of Cleavage: A- Holoblastic : complete or entire cleavage as in case of isolecithal and Mesolecithal eggs. B- meroblastic : incomplete or partial cleavage as in case of Telolecithal and Centrolecithal eggs. C- Placentals: the cleavage in placental mammals( Alecithal egg).

D- Determinate: Blastomere produced by early embryonic cleavage does not have the capacity to develop into a complete embryo. E- Indeterminate: The original cell (zygote) divides and the two resulting cells can be separated, and each one can individually develop into a whole organism.

Different Types of Eggs Eggs are classified into different groups according to their quantity & distribution of yolk, cytoplasm etc. The proportion of proteins and neutral fats varies and the yolk is accordingly known as 'protein yolk' or 'fatty yolk.‘ Classification Of Eggs Based Upon Amount Of Yolk Egg Type Representative Organisms A) Alecithal: Placental mammals Almost without yolk

Egg Type Representative Organisms B) Oligolecithal: Echinoderms, lower chordates Small amount of yolk (Amphioxus), marsupials C) Mesolecithal: Lung-fishes, frogs, and toads Moderate amount of yolk D) Polylecithal: insects, Sharks, bony fishes, Very large amounts of yolk reptiles and birds