Embryogenesis is a post-fertilization event with a series of changes throughout the period. It is one of the most significant during the 1st trimester. After fertilization, the product is a diploid zygote - conceptus (Diploid is the combination of genetic material from mother and father). During the initial period of embryo formation,  cell division occurs in a multiplication fashion. Cells undergo a systematic and rapid multiplication such as 2 celled product becomes 4 celled and 4 becomes 16 celled and so on (  refer the image below). The mass of cells eventually shaped into an embryo. Embryogenesis is the formation of a completely structured embryo from the undifferentiated mass of cells which usually takes place 4-8 weeks after the fusion of sperm and ovum. Post-fertilization, the process initially starts with the development of a zygote and then into the embryoThe process of development of an embryo from the zygote is called embryogenesis. There are 2 stages of development during the pregnancy. The first stage is called the embryonic phase and the later is the fetal phase. The zygote development depends upon various factors such as the environment, the nutritional condition of the mother, general health status, etc. 


1. Phases of embryogenesis

 1.1  phase-1 (Pre-implantation phase )

 1. 2 Phase 2 (Implantation phase)

2. Germ layer formation

3. Embryogenesis in Animals and birds

   Cell changes after fertilization        


Phases of embryogenesis

1. phase-1 (Pre-implantation phase )

After the fertilization, the conceptus(zygote and its associated membranes) move towards the uterus with the help of the ciliary peristalsis. During the journey, 5-6  rounds of cell replications happen within the conceptus. The daughter cells produced by such division is called a blastomere. At this stage, cells look like a compact solid mass known as morula( a bunch of mulberry like appearance). As the journey continues the ball of cells further multiply to form a mass of 100 cells.  The ball of cells starts to organize into what is known as blastocoel. The blastocoel secretes a fluid-like material to form a fluid-filled cavity within which the blastocoel fits in aWithin 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 from the inner yolk sac and an outer chorionic sac. Throughout the embryogenesis,  the embryonic cells receive their nutrition from the uterine and the ovarian canal.

2. Phase 2 (Implantation phase)

During the implantation,  the dividing embryo travels through the reproductive canal to settle inside the fallopian tubes. This is known as the implantation. It 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 it to adhere to the inner wall of the uterus. During this phase, women may experience slight bleeding within the reproductive canal; which can be mistaken as menstrual bleeding. About 50 –75% of dividing blastocyst fails to get implanted because of various reasons. When implantation succeeds there some, the trophoblast starts secreting the human chorionic gonadotropin (HCG). HCG stimulates the corpus luteum of the cellular mass to produce estrogen and the progesterone that suppress the menstruation. HCG is also important for furnishing the nourishment and shelter for the developing embryo. 

This figure shows the different stages in pre-embryonic development. A diagram of the uterus is shown and from this image, eight callouts show the different stages of development.

Pre-Embryonic Development


3. Germ layer formation

After the complete cell differentiation, the maternal hormones start actively functioning. Blastocyst is differentiated into many cavities, glands, membranes and various other structures necessary to form a primordial human. As early as the first month of embryogenesis embryo is reshaped into a 3 layered disc containing Endoderm, mesoderm and the ectoderm. Endoderm is an inner part that forms the 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. The oviparous is when the development of zygote takes place outside the female parent. In other words, egg-laying species follow this method. 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. This happens after the parent attains a certain stage of growth. After this development, the young ones are delivered out of the body of the female organism and in general, these are non-egg laying species.