Every organism is formed by a tiny embryo formed after the fertilization. An embryo is simply the organized mass of cells that is diffciult to identify due to poor differentiation. Embryogenesis is a post-fertilization event where  a series of cellular changes results in a very tiny and poorly differentiated individual. It is one of the most significant process that occurs during the first 3 months of the pregnancy (1st trimester). Followed by the fertilization of sperm and ovum, the zygote (conceptual product) is formed . A zygote is the diploid unit formed by consolidating the genetic material from both mother and father, hence it is genetically unique. During the initial period of embryo formation,  cell division occurs very rapidly forming the clumps of cells. During each phase of cell division, cells get multiplied like 2, 4, 16 etc. (refer the image below) while these dividing cells start searching for a safe location. The process of settling down of the rapidly dividing cellular mass is called implantation. 



Embryogenesis is the formation of a well-developed mass of cells called an embryo. Embryo is the product of undifferentiated mass of cells undergoing a rapid cell division which usually takes place between 4th-8th weeks after the fusion of sperm and ovum. The Post-fertilization is divided into 2 phases, embryonic and fetal phase where the process of development of an embryo from the zygote is called embryogenesis.  Embryonic phase is followed by the fetal phase which lasts between 6 weeks till the delivery. Let us understand bit more details about embryogenesis.

 Cell changes after fertilization        


Phases of embryogenesis

1. phase-1 (Pre-implantation phase )

After the fertilization, the conceptus(zygote and its associated membranes) start advancing towards the uterus with the help of the ciliary peristalsis. The cilia found in the ovaries play an important role in ciliary paristalsis. During this journey, the partly differentiated product undergoes at least 5-6  rounds of cell replications resulting in the formation of a blastomere. Blastomere continues its division to form a ball of cells that look-like a compact solid mass known as morula (a bunch of mulberries like appearance). As the journey continues, the ball of cells further multiply to form a mass of 100 cells while they simultaneosuly organized into what is known as blastocoel. The blastocoel secretes a fluid-like material while it is moulded into a fluid-filled cavity within which the blastocoel fits inWithin the blastocoel, the cells undergo distinctive differentiation to produce Embryo. The outer cells of the embryo form a shell known as trophoblast. Later on, trophoblastic cells are divided into an area, called inner yolk-sac and the outer chorionic-sac. During the embryogenesis,  the embryonic cells receive their nutrition from the inner wall of the uterus and the ovarian canal.


2. Phase 2 (Implantation phase)

The dividing embryo travels through the reproductive canal to settle inside the fallopian tube and this process is known as the implantation. The implantation takes approximately  5-7 days from the day of fertilization. In the initial phase of cell division, blastocyst comes in contact with the inner line of the uterus, simultaneously, the ciliary projections(trophoblastic cells) present on the surface of cellular mass help to adhere the dividing mass against the inner wall of the uterus. During this phase, women may experience slight bleeding in the reproductive canal; which can be mistaken as menstrual bleeding. Roughly, 50 –75% of dividing blastocyst fails to get implanted because of various reasons. In those cases where the implantation is succseful, the trophoblast ( settled product) starts secreting a hormone called human chorionic gonadotropin (HCG). HCG stimulates the corpus luteum(one of the layers of trophoblast)  to produce estrogen and the progesterone. Estrogen and progesterone suppress the menstruation resulting in missed periods; it is one of the indications of pregnancy. HCG is also important for furnishing the nourishment and shelter for the developing embryo. 


Pre-Embryonic Development



3. Germ layer formation

After the complete cell differentiation, the maternal hormones start functioning actively. Blastocyst is again differentiated into a number of cavities, glands, membranes and various other structures necessary to form the rudimentary form of human. As early as the first month of embryogenesis the embryo is reshaped into a 3 layered disc containing Endoderm, mesoderm and the ectodermEndoderm is an inner part of the mass that develops many vital organs and a middle Mesoderm layer forms bones, muscles, and the supportive structures whereas the outermost Ectoderm layer forms the protective sheaths of the fetus; skin and mucous membrane.


                 Pre-Embryonic Development


Embryogenesis in Animals and birds

Based on whether the development of the zygote occurs outside or inside the women`s body, the animals are divided into two categories namely, Oviparous and viviparous type. In the oviparous animals, the development of zygote takes place outside the female parent. To put it another way, these are egg-laying species  Once the fertilization is over, the fertilized eggs are covered with a hard calcareous shells in a safe place outside the body. Once the period of incubation is over, the young ones hatch out from the egg. In a Viviparous animal, the development of a zygote takes place within the body of the female parent. 



Read more


Contraceptive devices               


Reproductive health


Menstrual cycle


Human reproduction


Structure and functions of sperms                              



  1. Name the 3 important layers of embryo.

  2. What is a morula?

  3. Explain the process of implantation. 

  4. What are the different organs or body parts formed by ectoderm, mesoderm and endoderm?

  5. What is the role of HCG during pregnancy?

  6. Name  2 structures formed by the trophoblastic layer.

  7. Explain the important phases of embryogenesis





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