Question

Compare and contrast early embryonic development in amphibians, fish, birds, and mammals. Be sure to discuss specific features of development such as cleavage, gastrulation, and axis formation. Also, be sure to identify and discuss characteristics that are unique to each category of vertebrate.

Answer 1

Compare :In the early stage of development,the embryos of amphibians,fish,birds and mammals all look alike and similar.The  embryos in amphibians, fish,birds and mammals all possess gill arches, notochords, spinal cords, and primitive kidneys.All the embryos undergo the process of cleavage,blastulation and gastrulation to give rise to cells that undergo differentiation to form specialised organs.

Contrast:

• Cleavage : In amphibians,cleavage is holoblastic and bilateral.In fishes and birds and cleavage is meroblastic and discoidal while in mammals ,the cleavage is holoblastic.
• Gastrulation:In amphibians,gastrulation starts with invagination of blastula that gives rise to inner layer of cells i.e endoderm and an outer layer of cells i.e ectoderm. The third layer which is in theb middle is formed as cells begin to be formed in the the space between the endoderm and the ectoderm.In birds and mammals, the process of gastrulation is such that the epiblast cells tend to converge at the midline of the embryo and then move inward towards the primitive streak. It is the Ingression of these cells that leads to mesoderm formation and replacement of hypoblast cells to form endoderm.In fishes,ingression or involution of the deep layer of cells takes place to form epiblast,hypoblast and yolk synticial layer that give rise to ectoderm,medoderm and endoderm respectively.
• Axis formation:In mammals and birds, axis formation occurs at the mid line of the embryo.In amphibians and fishes, dorsal ventral axis formation takes place in embryo.