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IMPLANTATION, FETUS ETAL MEMBRANES AND Implantation is the embedding of the developing embryo m the lining of the uterus. In cow implantation actually begins from 11th to 40th day post coitum. After fertilization the zygote travels slowly down the oviduct; cell division occurs and this early cleavages of the fertilized egg are completed in the oviduct, the young embryo, consisting of from 8 to 16 cells (the blastocyst stage arrives in the uterus in search of permanent attachment by about 4 days. During the first days after its arrival in the uterus, the embryo is completely dependent upon uterine secretions for its energy. The uterine glands under the influence of oestrogen and progesterone hormone secrete 'uterine milk" which is composed of protein, fat and traces of glycogen. As the blastocyst increases in size, it can no longer absorb enough nutritive material by diffusion, and thus makes it imperative that it establishes a more adequate source of nutrition. About the 8 day zona pellucida begins to break up, and the cells push outward. Layers of cells form and from these layers grow membranes that soon will nourish the developing embryo. To understand the development of these membranes, it is first necessary to have some concept of the three primary germ layers in the developing embryo. After numerous cell divisions, the embryo is a hollow sphere of cells a few layers thick. This single tissue layer is known as the ectoderm, and is the origin of the skin and other structures. Somewhat later, one side of this sphere is pushed in to form a double walled cup-like structure, leaving ectoderm on the outer surface; the inner surface is known as endoderm, Later part of this pinches off into a tube to form the digestive tract. Proliferation of cells between the ectoderm and endoderm gives rise to a third germinal layer; the mesoderm. The mesoderm plays an important part in the formation of muscles. From these three germinal layers arise not only the various tissues of the developing embryo itself, but also those surrounding membranes, which protect it and enable it to obtain nourishment. These membranes are collectively known as the extra-embryonic membranes. The period of the embryo between 13th day to 45th day is characterised largely by the first formation of most organs and body parts. During this period the digestive tract, the lungs, the liver, and the pancreas all develop from the primitive gut. The heart and the circulatory system are started, and between 21st and 22nd day the heart begins to beat. The beginnings of the nervous system, the muscular and skeletal systems and the urogenital system are established. From the sides of the embryo a fold grows up and over it, fusing at the top and ultimately enclosing the embryo in a double-layered sac that is known as the amnion. This amnion or water bag as it is commonly called becomes filled with a clear watery fluid in which the embryo is suspended. Its purpose is to form a protective cushion against eternal shocks and pressure of the adjacent body organs and to prevent adhesions between the surface of the embryo and the surrounding membrane. At parturition, the amnion acts as a wedge to dilate the cervix% at which time it usually ruptures, allowing the "waters" to escape. As an outpouching of the hindgut of the digestive tract the allantois is formed. It enlarges and fills the space between the amnion and the outer membrane of the placenta, the Choron. The allantois functions as the urinary receptacle for the embryo and, also, collects some solid wastes. The chorion, .the outer membrane, completely surrounds the embryo, arnion and allantoic cavity, is rich in blood vessels and lies in opposition to the uterine mucosa, through which, by diffusion and osmosis, an exchange of gases and nutrients occurs between the blood vessels of the foetal circulation and the blood stream of the mother. The allantois which fuses with the chorion becomes the foetal placenta. The transfer of O2 CO2 and nutrients is affected by more or less intimate union of the chorion with the uterine mucosa of the dam to form the placenta. The mucosa, or lining of the uterus is essentially a spongy network of blood sinuses in which the finger-like villi of the chorion bury themselves. Thus, the placenta is partly maternal and partly foetal in its origin. There is no mixing of maternal and foetal blood, however, for both nutrients and gases must pass through the membrane of the placenta The food materials leave the maternal placenta and enter the foetal circulation by diffusion through the maternal and foetal placentae, after which they enter the blood vessels of the allatois witch terminate in the umbilical cord. Wastes are carried back to the maternal circulation by the same system. There are two general types of placentae among farm animals. The sow and the mare have a diffuse placenta; the entire chorion is beset with finger-like villi which fit into corresponding depressions in the uterine mucosa- The cow and the ewe have a cotyledonary placenta; the villi are localised in a hundred or so rosettes, or cotyledons, over the surface of the chorion. These are separated by areas of smooth chorion. The villi of the cotyledons fit into pits in the spongy button like cotyledons of the uterus. At parturition, the chorionic villi of both types are merely withdrawn, and there is no extensive destruction of the uterine tissue. The extent to which a foetus grows in the latter stages of pregnancy is demonstrated by the actual ·weights obtained by the late Sir John Hammond in one of his investigations carried out on Shorthorn heifers: Table Month of pregnancy 1 2 3 4 5 6 7 8 91/2 Weight of foetus Or calf (kg.)-- 0.00009 0. 009 0. 090 0.730 1-816 2.724 10.442 16.80 34.00 Correct feeding is important throughout pregnancy, but it is even more important during the last stages, when the foetus is making very rapid growth.