Answer
The three stages of human development before birth are the pre-embryonic stage, the embryonic stage, and the fetal stage:
1. Events of the pre-embryonic development period:
Ovulation occurs and one sperm fertilizes the ovum-- in an oviduct. Following this event, cleavage divisions occur that transform the zygote into a very small multi-cellular ball of cells --the morula. Later in this first week of development, the morula changes from a solid ball of cells into a more complex structure called a blastocyst. The blastocyst is still a very small sphere of blastomeres, but it forms structures including an inner cell mass and an outer layer of cells. The blastocyst also develops a cavity ( the blastocoel) which accumulates fluid. The chorion, one of the extra-embryonic membranes, which is formed from the trophoblast, also make its appearance at this time.
2. Events of Embryonic development:
The process of implantation begins in the second week of human development. Fertilization takes place in the uterine tube, but implantation takes place in the endometrium of the proximal part of the body of the uterus. The next structures to form are the amnion and yolk sac. The beginning of placenta formation marks the end of the second week of development
Placenta formation continues in the third week; also, the fourth extra-embryonic membrane--the allantois-- blood vessels, and nervous tissues develop. By the end of this week the placenta is well formed.
The embryo grows limb buds and develops a tail in the fourth week. In addition, during this period, a beating heart becomes apparent, nervous system growth progresses, and the embryo twists and changes its shape.
At the the beginning of the fifth week the curved embryo has a large head on which nose, eyes and ears are noticeable; the limb buds also develop divisions.
By the sixth week, fingers and toes develop from the limb buds; the skeleton is in place, but it is completely cartilaginous -- ossification takes place later.
By the end of the second month the uterus is large and is palpable above the pubic bone. Much happens in the seventh and eighth weeks: all systems are established by the end of this period; ossification of the skeleton progresses, and facial features are beginning to seem/appear human.
Work Step by Step
After release of an ovum, fertilization may take place --usually in a uterine tube. Because of the blocks to polyspermy, generally, only one sperm succeeds in fertilizing of the ovum. After fertilization, the zygote carries out a number of mitotic cleavage divisions while travelling down the oviduct on the way to the uterus. Although the morula is composed of many cells (16-60 or more) for some time it is no bigger than the zygote because the blastomeres are very small.
As cell divisions continue the morula changes shape and structure to form a blastocyst. A blastocyst comprises an inner cell mass, with an outer layer of cells--the trophoblast--and a fluid filled cavity --the blastocoel --the embryo will develop from the inner cell mass and the chorion will develop from the trophoblast.
Embryonic development. By the beginning of the second week of development the blastocyst is in the upper part of the uterine and is being nourished by the uterine fluid for a while before implantation. Implantation takes place in the wall of the proximal part of the uterus --closer to the fundus.; this usually happens about the end of the first to the beginning of the second week. The embryonic stage of development starts at this time and continues until the end of the eighth week --after fertilization. Implantation establishes pregnancy.
After implantation the chorion secretes human chorionic hormone (hCG) which stimulates the corpus luteum to produce progesterone. Progesterone maintains the endometrium so that menstruation ( loss of endometrium) does not occur, and the myometrium is calmed so it does not epel the conceptus. Late in the second week, the inner cell mass forms the embryonic disc; two other extraembryonic membranes-- amnion and yolk sac also develop at this time.
Gastrulation also occurs in this period: the inner cell mass morphs into the embryonic disc which gives rise to the three primary germ layers --ectoderm, mesoderm and endoderm.
Several organ systems appear in the third week after fertilization: the nervous system is the first to appear--neural folds .neural tube : the neural tube will later give rise to the central nervous system. The heart and blood vessels also appear in this week.
In the fourth and fifth weeks the body stalk (future umbilical cord) connects the chorion to the embryo. Also, the fourth extraembryonic membrane, the allantois and its blood vessels develop within the body stalk. Additional changes include the anterior movement of the body stalk as the head and neck lift up above the flat plane of the embryonic disc. The umbilical cord connects the placenta with embryo.
The next structures to appear--in week five--are limb buds from which arms, legs and feet and hands will develop. In this period, too, the head enlarges and eyes, nose and ears become apparent.
The seventh to the eighth week marks the end of the period of embryonic development. At the end of this period, all organ systems are present and established; the nervous system is functional and reflex reactions are possible . The size of the conceptus at this time is about 38-40 mm long.
Development after this period is subsumed/covered under the stage of fetal development.
