As the blastocyst develops further, it gives rise not only to the tissues and organs of the embryo, but also to a number of structures that support the embryo and help it to acquire nutrition. At a very early stage in development, the embryo proper acquires the form of a three- layered disc. This is called the embryonic disc (also called embryonic area,embryonic shield,or germ disc)The three layers that constitute this embryonic disc are.
(i) Endoderm (endo = inside)
(ii) Ectoderm (ecto =outside)
(iii) Mesoderm(meso = in the middle) These are thethree germ layers. All tissues of the body are derived from one or more of these layers. We have seen that the blastocyst is a spherical cyst lined by flattened trophoblastic cells, and that inside it there is a mass of cells,the inner cell mass, attached eccentrically to the trophoblast. Further changes are as follows.
(a) Some cells of the inner cell mass differentiate( i.e. they become different from others) into flattened cells, that come to line its free surface (fig. 8A). These are called hypoblast and constitute theendoderm, which is thus the first germ layer to be formed.
(b) The remaining cell of the inner cell mass become columnar (fig. 8B).These cells are called epiblast and form the second germ layer, the ectoderm. The embryo is now in the form of a disc having two layers.
(c) A space appears between the ectoderm (below) and the trophoblast (above). This is theamniotic cavity (fig. 8C), filled byamniotic fluid, or liquor amnii. The roof of this cavity is formed byamniogenic cells(cells of Rauber) derived from the trophoblast, while its floor is formed by the ectoderm.
(d) Flattened cells arising from the endoderm, spread and line the inside of the blastocystic cavity. In this way,a cavity, lined on all sides by cells of endodermal origin, is formed.This cavity is called theprimary yolk sac (fig. 8D).
(e) The cells of the trophoblast give origin to a mass of cells called the extra- embryonic mesoderm (or primary mesoderm). These cells come to lie between the trophoblast and the flattened endodermal cells lining the yolk sac, thus separating them from each other. These cells also separate the wall of the amniotic cavity from the trophoblast (fig. 9A).
This mesoderm is called extra-embryonic because it lies out side the embryonic disc. It does not give rise to any tissues of the embryo itself.
(f) Small cavities appear in the extra- embryonic mesoderm. Gradually these join together to from larger spaces and, ultimately, one large space is formed. This cavity is called the extra-embryonic coelom (fig. 9B). With its formation, the extra- embryonic mesoderm is split into two layers. The part lining the inside of the trophoblast, and the outside of the amniotic cavity, is called the parietal or somatopleuric extra-embryonic mesoderm.
The part lining the outside of the yolk sac is called the visceral or splanchnopleuric extra-embryonic mesoderm (fig. 9B), it will be seen that the extra-embryonic coelom does not extend into that part of the extra-embryonic mesoderm which attaches the wall of the amniotic cavity to the trophoblast. The developing embryo, along with the amniotic cavity and the yolk sac, is now suspended in the extra-embryonic coelom, and is attached to the wall of the blastocyst (i.e.trophoblast) only by this unsplitted part of the extra- embryonic mesoderm. This mesoderm forms a structure called the connecting stalk.
(g) Formation of chorion and amnion : At this stage, two very important membranes are formed. One is formed by the parietal extra- embryonic mesoderm (on the inside) and the overlying trophoblast (on the outside) this is called thechorion (fig. 9B). The other is theamnionwhich is constituted by the amniogenic cells forming the wall of the amniotic cavity (excluding the ectodermal floor). These cells are derived from the trophoblast. We have already seen that the amnion is covered by the unsplit extra-embryonic mesoderm, and that the connecting stalk is attached to it.
(h) With the appearance of the extra- embryonic mesoderm, and later of the extra-embryonic coelom, the yolk sac becomes much smaller than before and is now called thesecondary yolk sac. This alteration in size is accompanied by a change in the nature of the lining cells. They are no longer flattended but become cubical (fig. 9B).
(i) At this stage, the embryo proper is a circular disc composed of two layers of cells : the upper layer (towards amniotic cavity) is the ectoderm, the cells of which are columnar, while the lower layer (towards yolk sac) is the endoderm, made up of cubical cells (fig. 10).
(j) At one circular area a near the margin of the disc, the cubical cells of the endoderm become columnar. This area is called the prochordal plate. The appearance of the prochordal plate determines the central axis (antero posterier axis) of the embryo ( i.e. enables us to divide it into right and left halves) , and also enables us to distinguish its head and tail ends (fig. 10).
(k) Soon after the formation of the prochordal plate some of the ectodermal cells lying along the central axis, near the tail end of the disc, begin to proliferate, and form an elevation that bulges into the amniotic cavity. This elevation is called theprimitive streak (fig. 11). The primitive streak is at first a rounded or oval swelling, Primitive streak later forms the Henson's node.
(l) The cells that proliferate in the region of the primitive streak pass sideways,pushing themselves between the ectoderm and endoderm (fig. 11). These cells form the intra- embryonic mesoderm (or secondary mesoderm) which is the third germ layer.
Notochord - The notochord is a midline structure, that develops from mesoderm in dorsal region.
Importance of the notochord : The notochord is present in all animals that belong to the phylum Chordata. In some of them e.g. Amphioxus, it persists into adult life and forms the central axis of the body. In others, including man, it appears in the embryo but only small remainants of it remain in the adult. Notochord elongates considerably,and lies in the midine, in the position to be later occupied by the vertebral column. However, the notochord does not give rise to the vertebral column.
Neurulation :i.e. formation of neural tube. After neurulation there occur three type of ectoderm.
(i) Somatic ectoderm
(ii) Neural tube ectoderm
(iii) Neural crest ectoderm.
Anterior part of neural tube differentiate in brain and rest of the neural tube differentiate in spinal cord so central nervous system is formed by neural tube.
During the first 3 months of pregnancy the basic structure of baby is formed.This involves cell division, cell migration, and the differentiation of cells into the many types found in the body. Du