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

Cleavage :

Meroblastic cleavage is a type of cleavage that occurs in very yolky eggs like birds in which the egg cytoplasm and not the yolk divides (incomplete division). There are two types of meroblastic cleavage discoidal and superficial. Discoidal usually occurs in birds, reptiles, fishes etc.

Gastrulation: The fertilized zygote contains all the instructions for its
embryonic development in the zygotic genome and as
maternally derived cytoplasmic substances. The relative contributions of zygotic and maternal regulation
vary among vertebrates and this is reflected particularly in the speed and pattern of the early cleavages,
and consequently in the morphology of the blastula.
Fish and amphibian embryos develop externally and
the fast rate of their early development ensures swift
formation of independent larvae. These embryos rely
on rich energy stores in the form of a yolk and on
maternal determinants that mediate development until
the midblastula stage, when the zygotic genome
becomes transcriptionally active and takes control. The yolk is generally concentrated vegetally and
is either distributed between the blastomeres via complete cleavages, as in frog embryos, or deposited in a
separate yolk cell, as seen in incompletely cleaving
fish embryos.Consequently, the fish
blastula is a mound of blastomeres at the animal
region on top of a large syncytial yolk cell

By contrast, the frog blastula consists of smaller blastomeres at the animal hemisphere surrounding a blastocoel cavity and larger blastomeres in the vegetal region.

Axis Formation:

Ambhibians-

In Xenopus (and other vertebrates), the formation of the anterior-posterior axis follows the formation of the dorsal-ventral axis. Once the dorsal portion of the embryo is established, the movement of the involuting mesoderm establishes the anterior-posterior axis.

Fish: Once the egg has become multicellular and positioned its germ layers with ectoderm on the outside, mesoderm in the middle, and endoderm on the inside body axes have to be determined for proper development. A dorsal-ventral axis has to form and major proteins involved are BMP and Wnts.

Birds : The primitive streak defines the axes of the embryo. It extends from posterior to anterior; migrating cells enter through its dorsal side and move to its ventral side; and it separates the left portion of the embryo from the right.

Mammals:

The dorsal-ventral axis

Very little is known about the mechanisms of dorsal-ventral axis formation in mammals. ... Thus, the dorsal-ventral axis of the embryo is, in part, defined by the embryonic-abembryonic axis of the blastocyst.