SS2 Biology Lesson Note on Structural Differences in Eggs of Vertebrates

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TOPIC: REPRODUCTIVE SYSTEM IN VERTEBRATES II

CONTENT:

1. Structural differences in Eggs of vertebrates.
2. Comparison of reproduction in fish, reptiles and mammals.

SUB-TOPIC 1: Structural differences in Eggs of vertebrates.

The fish eggs are very small and appear like mass of sand grains. The egg contains the young fish and is enclosed in and egg case or membrane. In amphibians- toad or frog, the eggs are small and spherical. An egg consists of semi liquid cytoplasm containing a nucleus surrounded by a tough black egg membrane. The toads egg is black on the under surface because the protoplasm of the egg is full of yolk granules abundant in the lower part of the eggs.

The egg has a thin coat which absorbs water and swells to form the jelly or albumen. This jelly protects the egg and separates them from one another in a bead-like manner.

In reptiles, e.g., Agama lizard. The egg is cream coloured. It has a soft but tough leathery shell. It absorbs water from the surrounding soil and increase in size or volume.

The egg of the bird is the largest single animal cell. Its porous shell allows for the exchange of gases with the egg and its environment. The egg possesses two membranes enclosing the albumen. The albumen contains the yolk in which the germinal disc or embryo occupies the inner most core.

The mammalian egg is microscopic and is about 0.1mm in diameter. It consists of the cytoplasm, a nucleus in the centre, granules and yolk droplets. The yolk provides a source of nourishment for developing embryo. The ovum (egg) is surrounded by two membranes. The inner one is the plasma membrane while the outer one is the vitelline membrane.

Life Cycle of a Jellyfish

In the reproductive life cycle of a typical jellyfish, males release sperm and females release eggs into the water. When an egg and sperm fuse to form a fertilized egg during sexual reproduction, a larva develops that attaches to a rock or other object and develops into a polyp. In a type of asexual reproduction, the polyp transforms into a colony of polyps that resembles a stack of saucers. Each saucer in the stack detaches itself from the colony as a new medusa, and the reproductive cycle repeats.

EVALUATION

1. Mention two structural differences between the eggs of a fish and a reptile.
2. In what ways are the eggs of a mammal different from that of reptile?

SUB-TOPIC 2: COMPARISON OF REPRODUCTION IN FISH, REPTILES AND MAMMALS.

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