Baby Development: 2 Week Old

Second week of development
The second week of embryonic is characterized by the following:

• The completion of implantation of the blastocyst and further development of the trophoblast.
• Formation of bilaminar embryonic disc composed of two distinct layers epiblast and hypoblast.
• Development of an amniotic cavity and yolk sac in association with these layers.

Eighth day
A) formation of bilaminar disc:
Cells of inner cell mass or embryoblast differentiate inti two layers:
(i) Hypoblast: small cuboidal cells adjacent cavity.
(ii) Epiblast: High columnar cells adjacent to amniotic cavity.
Each germ layer forms a flast disc and together is known as bi laminar embryonic disc.

B) Further differentiation of trophoblast:
Trophoblast embeds itself into endometrium and two complete layers are fromed

• Cytotrophoblast (mononucleated later i.e mitotically active).
•  Syncytiotrophoblast (multinucleated without distinct cell boundaries * no mitosis occurs)

C) Formation of amniotic cavity:

     A small cavity appears with in the epiblast, which later enlarges to become amniotic cavity. As the amniotic cavity enlarges it acquires a thin epithlial domed shaped roof called the Amnion. The cells that form the amion  are called amnioblast.

Ninth Day
At 9th Blastocyst is more deeply embedded in the superficial later of endometrium and approximately at 10th day the penetration defect is closed by fibrin coagulum.
A) Formation of primary yolk sac:
    As the amnion is formed, other cells also arise from the cytotrophoblast and form a thin Exoceolomic Membrance or Heuser's Membrane. This membrance is continuous with the huppblast of the embryonic disc & forms the wall of the primary yolk sac or exocoelomic cavity.

B) Formation of uteroplacental circulation:
     As the amniotic cavity   & primary yolk sac are formed, isolated cavities (spaces) or Lacunae appear in between syncytiotrophoblast. These lacunae are filled bya fluid (sometimes called Embryotroph) composed of maternal blood, ruptured endometrial blood vessels and secretions from uterine glands. The maternal blood in the lacunae also receives hCG produced by the corpus luteum. this communication of eroded endometrial capillaries with lacunae reresents the corpus luteum. This communication of eroded endometrial capillaries with lacunae represents the beginning of uteroplacental circulation.

Tenth toTwelveth Day
A) Formation of extraembryonic mesoderm & extraembryonic coelom:
    As delamination of cytotrophoblast occurs, the cells of yolk sac endoderm give rise to a layer of lossely arranged connective tissue called extraembryonic mesoderm. It completely fills the space between the amniotic cavity. yolk sacs and the trophoblast. As the changes occur in the tropho blast & endometrium the extraembryonic mesoderm increase and isolated spaces appear in it. These spaces rapidly fuse to form a single large cavity called extraembryonic coelom. This coelom splits the extrambryonic mesoderm into somatic (lines the trophoblast) and splanchnic ( surrounds the yolk sac) layers.
Extraembryonic coelom surrounds the amnion & yolk sac except where they are attached to the chorion by connecting stalk ( a slender band of extraembryonic mesoderm and primordium of umbilical cord.

B) Formation of chorion & chorionic cavity:
     The extraembryonic mesoderm and two layers of trophoblast conctitute the chorion. The chorion forms the wall of chorionic cavity with in which the embryo its amnion & yolk sac are suspended by the connecting stalk. the extraembryonic coelom is now called chorionic cavity.

Thirteen Day .
The Exocoelomic cavity is pinched off and is represented by excoelomic cst. As pinching occurs the primitive yolk sac is reduced in size and known as secondary yolk sac. this yolk sac contains no yolk but is incilved in the trn asfer of nutrient and oxygen to the embryo from the maternal blood.

A) Foramtion of chotionic villi:
     Primary chorionic villi develop as outgrowths of trophoblast from the chorion. They consist of a core of cytotrophoblast covered by a thick layer od suncytiotrophoblast. these primary villi are primordia of the chorionic vili of the placenta.

B) Prochordal plate:
     It is the thickening of hypoblast cells in head region and indicates where the mouth of the embryo will develop.

Baby Development: 1 Week Old

Cleavage
As the zygote passes along the uterine tube to the uterus it indergoes cleavage i.e a series of rapid mitotic cell divisions. The new cells are known as Blastomers. After several divisions a mulberry shaped mass of 16 blastomeres is formed which is know as morula. It is surrounded by a protective layer known as the zona pellucida ( secreted by granulosa cells).

Third to Fourth day
A) Blastocyst formation:
     Three days after fertilization the morula enters the uterus. Uterine fluid passes through the zona pellucida, and fills the spaces, which appear between the central cells of the morula. these spaces soon coalesce to form a blastocyst cavity and thus developing human is called Blastocyst.

B) Differentiation of blastocyst cells:
    As the amount of fluid in the blastocyst cavity increases, the cells becomes separated into two parts.
(i) Trophoblast or outer cell mass: It is flattened and eventually gives rise to embryonic part of the placents.
(ii) Embryoblast or inner cell mass: It is centrally located and is the primordial of the embryo.

Fifth Day
A) Degeneration of Zona pellucida:
     About five days after fertilization the zona pellucida disappears and the blastocyst enlarges.

Sixth day
A) Initiation of implantation:
     The trophoblast attaches to the endometrial epithlium to initiate the provess known as Implantation.
B) Differentiation of trophoblast:
     The attached region of the trophoblast differentiates into two layers cytotrophoblast (the inner cellular layer) and syncytiotrophoblast (an external syncytial layer), which invade the endometrial epithelium and connective tissue by the and of the seventh day.

Seventh day
A) Formation of hypoblast:
    By the end of the first week a layer of cuboidal cells known as the hypoblast is formed on the ventral surface of the inner cell mass. It given rise to the primitive endoderm.