Cells from the cell mass will form a lining, developing the first of four extra-embryonic membranes, the amnion or amniotic sac, which provides a fluid-filled space that acts as a protective cushion, permits symmetrical, growth, helps the embryo control body temperature, prevents adherence of the embryo to the amnion and protects against infection. The two cell layers of the cell mass separate, creating a cavity which will become the amniotic cavity.The nutrients released from these cells are used to provide food for the developing pre-embryo. The breakdown of the cells of the uterine lining (endometrium) to form a place for implantation is accomplished by the action of the enzyme hyaluronidase-the same enzyme the sperm uses to get to the egg for fertilization. By the 4th or 5th day of development, the developing cell mass or pre-embryo becomes implanted in the lining (implantation).These individuals are joined at the spot where the division of the blastula was not completed. If the division is not complete, conjoined twins (Siamese twins) develop. It is at this time that identical twins (monozygotic twins) may form as a result of the blastula dividing completely in two. By the end of the week, this ball has become hollow (a blastula) with a swelled mass of cells on one side (cell mass).Fertilized egg begins to divide (cleavage), that is 1 cell divides into 2, which divide into 4,8.16, and so on until a ball of cells (morula) is formed.Read about the first 12 weeks of human development, week by week, and see what happens to your baby! Week 1 During the first trimester of pregnancy, major changes take place in fetal development.
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