Embryonic Development

In human beings, during the fourth week after fertilisation, the embryo develops a simple heart.

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Metamorphosis is a profound change in form from one stage to the next in the life history of an organism, as from the caterpillar to the pupa and from the pupa to the adult butterfly.

Excretory products of mammalian embryo are eliminated by placenta.

The solid ball of cells ,resembling mulberry fruit ,resulting from the cleavage of zygote  is called morula .

Central placentation is also known as free placentation. Here, the placentae are arranged in a central column within a non-sectioned ovary and is seen in compound carpel.

The tap root system develops from the radicle (embryonic root) of the seed, making it embryonic in origin. Fibrous roots develop from the base of the stem (adventitious), and prop roots are adventitious roots providing support.

The radicle is the embryonic root in the seed that emerges first during germination and develops into the primary root system. The plumule becomes the shoot, hypocotyl is the region below the cotyledonary node, and cotyledons are seed leaves.

In a monocarpellary (one carpel), unilocular (one chamber) ovary, ovules are arranged along the ventral suture where the carpel margins fuse. This marginal placentation is characteristic of legumes. Axile is for multicarpellary ovaries, parietal for syncarpous ovaries with false septa, basal for single basal ovule.

Marginal placentation (a) develops from bicarpellary, syncarpous ovary that becomes unilocular (as in Fabaceae). Parietal placentation (c) develops from multicarpellary, syncarpous, unilocular ovary where ovules attach to the ovary wall. Basal placentation (d) develops from bicarpellary, syncarpous, unilocular ovary with single ovule at base. Axile placentation (b) requires multilocular ovary, not unilocular.

Axile placentation occurs when ovules are attached to a central column formed by the fused margins of carpels. In a pentacarpellary (5 carpels), syncarpous (fused carpels), and pentalocular (5 chambers) ovary, this central arrangement is exactly what creates the axile pattern. Parietal placentation would give a unilocular ovary, while marginal occurs only in monocarpellary conditions.

Parietal placentation characteristically occurs in multicarpellary, syncarpous ovaries where the ovules are borne on the inner walls (periphery) of a single chamber (unilocular). The fusion of carpels creates one large cavity with ovules attached to the walls, unlike axile placentation which has multiple chambers.

 Amniotic sac is protected to the inner side by a membrane called amnion and towards the outer side is protected by a membrane called chorion.

 Gastrula stage follows after blastula and during this stage the cells of blastula develop into a tri-laminar structure and the three germ layers namely ectoderm,mesoderm and endoderm develop.