Gastrulation:

        The transformation of the blastula into the gastrula with three germ layers by the rearrangement of the cells is called gastrulation.

        Gastrulation involves cell movements called morphogenic movements.

        As a result of these cell movements three primary germinal layers, namely endoderm, mesoderm and ectoderm are formed. Gastrulation begins with the formation of blastopore and ends with the development of neural tube.

Formation of Endoderm:

        Cells of inner cell mass in contact with blastocoel flatten, divide and grow to form a complete layer around blastocoel.

        The layer is called endoderm. It is first germinal layer that appears in human embryo.

        It forms an endodermal tube/archenteron or primitive gut and its cavity is called gastrocoel.

        It opens to the outside through blastopore which later on closes.

        Blastopore is absent in amnions.

        Endoderm located under the embryonal knob is the embryonic endoderm.

        It later forms the true, or embryonic gut.

        The remaining endoderm along with the trophoblast forms the primary yolk sac.

        It communicates above with the true gut.

Formation of embryonic disc:

        After tie formation of endoderm, the embryonal knob is stretched and its cells become columnar and come to lie as a regular layer called embryonic disc.

        The embryonic disc consist of three regions, cephalic margin, and embryonic disc proper and caudal region. Future embryo develop from embryonic disc proper while the rest of the form extra embryonic mesoderm

Formation of Endoderm:

        In certain groups of animals, during the formation of embryonic digestive system, the first opening developed is mouth the anus develops later. These are called Protostomes. . Annelida, Mollusca, Arthropoda.

        In other groups of animals, e.g. echinoderms, vertebrates including man, during the formation of embryonic digestive system, anus develops first and the mouth is formed later. These group of animals are called deuterostomes i.e. blastopore of gastrula develops into anus.

Formation of Ectoderm:

        After formation of endoderm cells, the embryonic disc grows to produce a complete layer below the trophoblast it is ectoderm.

Formation of Amniotic Cavity:

        A space develops between ectoderm and trophoblast is called as amniotic cavity.

        Trophoblast adds special cells to its roof.

        They are called amniogenic cells.

        Amniotic cavity is filled with amniotic fluid.

        It acts as shock absorber.

Formation of Mesoderm:

        At the caudal region of embryonic disc, the cells start proliferation at an increased rate.

        A new layer of cells so formed is called mesodermal cells.

        These cells occupy the extraembryonic space between trophoblast, amnion and yolk sac.

        The cells form a layer around amnion and yolk sac called splanchnic mesoderm.

        The cells forms the layer below the trophoblast called somatic mesoderm.

        The space between two mesodermal layers is called extraembryonic coelom.

        Thus the first mesoderm to make its appearance is embryonal and embryonal disc still has ectoderm and endoderm only.

Formation of primitive streak and intra embryonic mesoderm (Mesoderm proper):

        It is the last stage of gastrulation.

        As the development proceeds, some endodermal cells along a longitudinal axis become columnar to form a protochordal plate which determines the central axis of embryo.

        The cells of the ectoderm in the central axis now proliferate at the future posterior end and form an elongated ridge called primitive streak on the 15th day after fertilization.

        Primitive streak is an elevated ridge projecting into the amniotic activity.

        The cells in the region of primitive streak, now divide and proliferate in the space between ectoderm and endoderm forming the third germ layer called intra embryonic mesoderm.

Significance of Gastrulation:

        1. It develops three germ layers.

        2. It develops archenteron which acts as future gut of organism.

        3. It slows down the cleavage rate.

        4. It increases the metabolic activities without increasing the size of the cells.

        5. Protochordal plate determines the central axis of embryo.

        6. Primitive streak marks the posterior end of the embryo.

        7. It develops different organs in the embryonic development.

Later development:

        Gastrulation is only the earliest stage of development.

        The embryo or foetus undergoes development for many more weeks, getting nourishment from the mother through placenta. This period is called pregnancy.

        This period of pregnancy from fertilization to parturition is called gestation period.

        In human beings the gestation period is about 280 days (40 week/9month 10 days) from the beginning of the last menstrual period.

Fate of Three Germ Layers:

        1. at the end of gastrulation three germ layers are formed Le. Ectoderm, mesoderm and endoderm.

        2. By the process of organogenesis various organ system are formed in embryo.

        3. The development of tissue and organs from the three germ layer is called organogenesis.

 

VERY SHORT ANSWER QUESTIONS

1.        What is gastrulation?

2.       Define gastrulation?

3.       What is primitive gut or archenteron?

4.       What is gastrocoel?

5.       Which cell lining is present with gastrocoel?

6.       Which cells develop yolk sac?

7.       Ho embryonic disc is formed?

8.       What is amniotic cavity? What does it contain?

9.       Which cells form amniotic cavity?

10.    What are the two layers formed by extra embryonic mesoderm?

11.     What is histogenesis?

12.    Name the different organs produced by ectoderm.

13.    Name the different organs produced by mesoderm.

14.    Enlist the different organs of endodermal origin.

15.    What are monozygotic or identical twins>?

16.    What are dizygotic or fraternal twins?

17.    What can be the sexes of dizygotic twins?