Question

1. Explain the primary differences between sea urchin and frog development during the processes of fertilization, cleavage, and gastrulation. In your answer, provide a basic description of each process, describe the developmental stage that results, explain the differences between sea urchin and frog embryos, and use the terms cortical reaction, cortical rotation, induction, and involution.

Fertilization:

Cleavage:

Gastrulation:

Answer 1

FERTILIZATION

-In sea urchin the mode of fertilisation is external i.e. the sperm and eggs are released in the water by male and female respectively. The sperm gets attracted by the chemicals released by the egg in the water ( chemotactic movement). This chemical (called resact) gives the sperm direction and increases the speed of its movement. This way, the sperm finds the egg Of similar species in the water. On fertilisation the egg, the sperm induces egg to undergo cortical action that is a method of slow black to polyspermy. It includes fusion of egg cell membrane with cortical granules and release of the content in between the cell membrane and vitelline membrane.water accumulated in this zone and forms a fertilisation cap.
- In most species of frogs, fertilization is external. The male frog grabs the female's back and fertilizes the eggs as the female frog releases them. Here the need for chemoattractant is less as the sperm is released right next to the eggs. After release, these eggs float in the water freely or are adhered to the a leaf by some jelly. Some species of frog carries their eggs with them.

CLEAVAGE

before I explain what type of cleave occurs in frog and sea urchin you should know the basic types of eggs and cleavage pattern. They are given below

- sea urchins have isolecithal eggs that shows holoblastic cleavage. The first and second cleavages are both meridional and are perpendicular to each other. The third cleavage is equatorial, perpendicular to the first two cleavage planes

- frogs have mesolecithal eggs that shows displaced radial holoblastic cleavage

GASTRULATION

- in sea urchin embryo gastrulation occurs in two phases- ingression of skeletogenic cells and then invagination of archenteron. It is explained in the diagram below.

the steps in gastrulation is described by another diagram below.

due to time constraints, I will explain the gastrulation in frog by diagram only. I will try to make it as clear as possible . Ask me any doubt in the comment section.

This rotation of the cortical pigmented cytoplasm on the sperm entry is called cortical rotation. It is ver Essential for the dorasal lip of blastopore formation that induces the secondary axis . The dorsal lip of blastopore forms the organiser and brings about primary embryonic induction.