Physiology : a manual for students and practitioners . d foetal portions arc so intermingled that they cannot beseparated. In size it covers about one-third of the uterine wall. At what period of gestation is the placenta formed ? At about the third month of pregnancy. Before that time tliochorion is covered by the decidua reflexa and nourishes the embryo,but as the placenta becomes more developed other parts of thechorion atrophy. From what tissues is the umbilical cord formed ? From the vascular allantois, whichcarries the arteries and vein. It has, Fig. 63. however, an external coating of t
Physiology : a manual for students and practitioners . d foetal portions arc so intermingled that they cannot beseparated. In size it covers about one-third of the uterine wall. At what period of gestation is the placenta formed ? At about the third month of pregnancy. Before that time tliochorion is covered by the decidua reflexa and nourishes the embryo,but as the placenta becomes more developed other parts of thechorion atrophy. From what tissues is the umbilical cord formed ? From the vascular allantois, whichcarries the arteries and vein. It has, Fig. 63. however, an external coating of theamnion and the shrivelled umbilicalvesicle and its duct (Fig. 63). Howthis occurs will readily be seen byreference to the accompanying cut. Why do the maternal vessels notbleed excessively after the pla-centa is torn from its implan-tation ? There is, of course, a loss of theblood contained in the uterine sin-uses, but the general balance of thecirculation is not disturbed at child-birth. The reason for this is the oblique entrance of the placental. Human Embryo anrt its Envelopes attlie I-nd of the Tliinl Month, show-ing the en hi rgeim-ut of the amnion. 192 EMBRYOLOGY. vessels. They enter the sinuses at an angle, and are thereforecompressed by the muscular tissue of the uterus in its contractedstate. How is the vertebral column developed? Early in the development of an embryo there is formed beneaththe medullary groove in the mesoblast a thin thread of soft carti-lage known as the chorda dorsalis, or notochord. This soon be-comes included in a sort of fibrous sheath, and is the primary axisfrom which the bodies of the vertebrae are developed. On eitherside of the notochord are developed small centres which subse-quently split. These are the protovertebrse. From these are de-veloped the vertebrae and the heads of the ribs by the inner lat/er,and by the outer (or posterior) lai/er the muscles and skin of theback, the epidermis being derived from the epiblast. How do the proto
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectphysiology, bookyear1
