[Biology Class Notes] on Embryo in Humans and Animals Pdf

Let’s know what an embryo is! The early developmental stage of an animal, while it is in the egg or within the uterus of the mother, is known as the embryo. In humans, the term embryo is applied to the unborn child until the end of the seventh week; from the eighth week, till the unborn child is known as a fetus.

In the organisms that generally reproduce sexually, the union of an ovum with a sperm results in the formation of a zygote, or fertilized egg, which undergoes a series of divisions known as cleavages as it passes down the fallopian tube. After several cleavages have taken place, the cells form a hollow ball which is known as a blastula. In most mammals the blastula attaches itself to the uterine lining, thus stimulating the formation of a placenta, which is responsible for transferring nutrients from the mother to the growing embryo. In lower animals, the yolk nourishes the embryo. 

Embryonic Stage in Animals

During the development of an animal embryo, its cells divide, grow, and migrate in specific patterns to make a more and more elaborate body (plant cells perform differential expansion instead of migration). To function properly, that body needs well-defined axes (such as head vs. tail). Embryos need a specific collection of many-celled organs and other structures, positioned in the right spots along the axes and connected up with one another in the right ways. How are all of these complex processes accomplished and coordinated? They occur via four essential stages in early animal development which we will discuss below:

• Fertilization: The process of a single sperm cell combining with a single egg cell to form a zygote is known as fertilization.

• Cleavage: The rapid, multiple rounds of mitotic cell division where the overall size of the embryo does not increase is known as cleavage. The developing embryo is known as a blastula following completion of cleavage.

• Gastrulation: The dramatic rearrangement (movement) of cells in the blastula to create the embryonic tissue layers is known as gastrulation. These tissue layers will go on to produce the tissues as well as organs of the adult animal.

• Organogenesis: The process of organ and tissue formation via cell division and differentiation is known as organogenesis.

The last two stages, gastrulation, and organogenesis, together contribute to morphogenesis: the biological processes that result in an organism’s shape and body organization.

Embryos of Different Animals

Embryos of many different kinds of animals: mammals, birds, reptiles as well as fishes look very identical and it is often difficult to tell them apart. It is known that many traits of one type of animal appear in the embryo of another type of animal. For example, fish embryos, as well as human embryos both, have gill slits. In fish they develop into gills, but in humans, they disappear before birth.

This basically shows that the animals are similar and that they develop similarly, implying that they are related, have common ancestors, and that they started out the same, gradually evolving different traits, but that the basic plan for a creature’s beginning remains indistinguishable.

Embryonic Formation Stage in Humans

These stages will help you understand the embryo meaning. The embryo is about the size of a pea at the age of four weeks. A primitive heart is beating, the head of the baby is defined with rudimentary eyes as well as ears, and tiny bumps represent arms and legs. The embryo also contains a primitive nervous system, and the head generally starts to enlarge. A cartilage skeleton has appeared, and the muscles of the child have taken shape.

By the end of eight weeks, the embryo starts looking like a human. Facial features are evident, and most of the organs are well developed by the end of eight weeks. From this point onward, the development consists chiefly of growth and maturation. The embryo is about 1.5 inches in length. Henceforth it is known as a fetus.

Nourishment of the embryo, and then the nourishment of the fetus, is accomplished through the placenta. The maternal, as well as embryonic blood supplies, meet at this organ, but the blood does not mix. Instead, diffusion is responsible for the passage of gases, nutrients, and waste products across the membranous barriers. 

The placenta is also an endocrine gland because it secretes estrogen and progesterone to continue to inhibit follicle development and maintain the integrity of the endometrium. As the embryo becomes a fetus, it moves away from the placenta, and a length of tissue known as the umbilical cord becomes its source of attachment to the maternal blood supply.

Types of Embryo

Based on the number of megaspores, we can classify the embryo sacs into three types: monosporic, bisporic, and tetrasporic.

What are the 4 Stages of Embryonic Development?

• Germinal stage. 1.1 Fertilization. 1.2 Cleavage. 1.3 Blastulation. 1.4 Implantation. 1.5 Embryonic disc.

• Gastrulation.

• Neurulation.

• Development of organs and organ systems.