Embryonic Development

In vertebrates skeleton muscles are mesodermal in origin. 

Adrenal cortex is derived from Mesoderm and Adrenal medulla is derived from Ectoderm.

Embryonic cells differenciate at an early stage and form an outer layer of cells, the ectoderm and an inner layer the endoderm. At the later stage the mesoderm is formed and these three germ layers give rise to all the tissues of the developing embryo. Kidney and ureter in Rana Tigrina is developed from mesodermal layer. 

Archenteron cavity is found in gastrula. It is a primitive or embryonic digestive tube and communicating with outside by blastopore. 

Gastrulation is the dynamic process involving critical changes in the embryo, such as differentiation of cells, establishment of the three primary germ layers and transformation of a single walled blastulla into a double walled gastrulla. During involution or emboly, the insinking endodermal cells migrate anteriorwards along the roof of blastocoel and give a new cavity called gastrocoel or archenteron. It represents the future gut cavity. Blastocoel formed during Blastulla stage is filled with an alkaline albuminous fluid secreted by surrounding micromeres.

Fraternal twins, or dizygotic twins, result from the fertilization of two separate ova by two separate sperm cells. Identical twins result from a single fertilized egg splitting.

Bicoid protein activates Hunch back gene.

Hunchback is a gap gene which is involved in first level of segmentation of the embryo.

The developing embryo does not receive nourishment from mother's body in oviparous reproduction.Oviparous animals are animals that lay eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, reptiles, all birds, the monotremes, and most insects, molluscs, and arachnids.

In mammals, the developing embryo is connected with placenta by umblical cord. During prenatal development, the umbilical cord is physiologically and genetically part of the fetus and, (in humans), normally contains two arteries (the umbilical arteries) and one vein (the umbilical vein), buried within Wharton's jelly.

The process by which blastocyst settles down on the uterine walls is known as blastulation.During blastulation, cells continue to divide and begin to differentiate.

 Embryology is the branch of biology that deals with the development of an embryo from the fertilization of the ovum to the fetus stage.

The developing embryo does not receive nourishment from mother's body in oviparous reproduction. 

Parthenogenesis is the development of an embryo from an unfertilised egg or if a spermatozoan does penetrate the egg, there is no union of male and female pronuclei. It is found in many plants (dandelions and hawk weeds) and animals (aphids and honey bees).

Cellular differentiation is the process by which a cell changes from one cell type to another. Usually, this is because a less specialised type becomes a more specialised type, such as during cell growth.