Sequential events and stages in the development of frog

Sequential events and stages in the development of frog (pre-
embryonic, embryonic and post-embryonic development)
Introduction:

 In sexual reproduction, development of organisms begins with a fertilized egg or zygote.
 Zygote divides and re-divides to form an embryo.
 Embryo is the earliest stage of development, while it is within the egg or reproductive organs of mother
until hatching or birth.
 The branch of biology dealing with the study of embryo is known as embryology.
 The development of embryo takes place in an orderly sequence and follows the same fundamentally
similar sequence of events in almost all sexually reproducing organisms.

Sequence of events before and during embryonic development in living organisms:

1. Gametogenesis: Formation of gametes or sex cells.
2. Mating: Transfer of male gametes inside female’s body by copulation.
3. Fertilization: Fusion of male and female gametes to form a zygote.
4. Cleavage: Repeated divisions taking place in the zygote.
5. Morulation: Formation of solid ball of cells.
6. Blastulation: Formation of hollow ball of cells.
7. Gastrulation: Movement and rearrangement of cells.
8. Organogenesis: Development and differentiation of different organs from primary germ layers.
9. Morphogenesis: Growth and differentiation of form and structure.
10. Growth: Increase in size and weight through cell division and cell enlargement.

Sequential events in the development of frog:

1. Gametogenesis:

 Sperms and ova (eggs) are produced by respective gonads in males and females by meiosis.
 The sperms are microscopic, thread like and motile, 0.03mm in length.
 Ova are larger than sperms, non-motile and nearly spherical. Eggs are mesolecithal and telolecithal.

, Sperm and ovum of frog

2. Mating:

 During breeding season (rainy season), the croaking of male attracts the female frog. Frogs copulate in
water by a sexual embrace called amplexus or pseudo-copulation.
 The male comes over the back of the female and firmly clasps her body by his forelimbs.
 When the female sheds ova (several hundred) through her cloaca, the male deposits spermatic fluid over
them and fertilization occurs.

3. Fertilization:

 Fertilization in frog is external, as it occurs in water.
 One sperm penetrates one ovum and as soon as the male pro-nucleus enters the ovum, a second polar
body is budded off from the ovum.
 The first polar body was already present below the vitelline membrane.
 After the second maturation division giving off second polar body, the fusion of male and female nuclei
takes place. This process is called fertilization.
 The cell thus formed is called zygote and the nucleus is called zygote nucleus which has diploid
chromosome number, 26.
 The zygote now divides through a process called cleavage or segmentation.

4. Cleavage or segmentation:

 In frog, the division is complete but the cells formed are unequal. So, the cleavage is
called holoblastic but unequal.
 The first division is vertical and divides the zygote into two blastomeres. The furrow extends from animal
pole to the vegetal pole (lower end).
 Second division is also vertical but at right angle to the first resulting in 4 blastomeres.
 The third division is horizontal passing above the equator forming 8 unequal blastomeres. Out of these, 4
upper smaller ones are called micromeres and 4 lower larger ones are called megameres.