Embryonic Development

Ans - 1

Frog eggs are mesolecithal, meaning they have a moderate amount of yolk. The cleavage is holoblastic, meaning the entire egg divides, though it is unequal.

The brain, spinal cord, pituitary gland, and pineal gland are all derived from the ectoderm during 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. 

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.

Larva of Ascaris first enter the host intestine and reaches the liver through portal system and lymph channel now it reaches to heart and then to lungs. In lungs, larva settles down in capillaries of alveoli for sometime and undergoes two moultings one after the other. 

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.