. Obstetrics for nurses. cells, which possess markedly invasive prop-erties, grow rapidly and constitute the chorion oroutermost membrane of the egg. In the earlyovum they are designated the trophohlast, whichthroughout pregnancy consists of fetal elements ^ ^^ T. • ,. which remain in direct contact with the tissues ofFig. 26. —Exterior of ^, . ^,,, . , . , ., , , early human ovum, the mother. Ibis chorion is pushed out l)y the Magnified four times, tissue (mesoderm) beneath it and soon forms(Carnegie Embryolog-ical Collection No. large numbers of long finger-like buds called villi, l^
. Obstetrics for nurses. cells, which possess markedly invasive prop-erties, grow rapidly and constitute the chorion oroutermost membrane of the egg. In the earlyovum they are designated the trophohlast, whichthroughout pregnancy consists of fetal elements ^ ^^ T. • ,. which remain in direct contact with the tissues ofFig. 26. —Exterior of ^, . ^,,, . , . , ., , , early human ovum, the mother. Ibis chorion is pushed out l)y the Magnified four times, tissue (mesoderm) beneath it and soon forms(Carnegie Embryolog-ical Collection No. large numbers of long finger-like buds called villi, l^^) which project from its exterior (Fig. 26). Even- tually the villi become extremely complicated ar-l)orescent structures. By the formation of such numbers of villi theabsorbing surface of the chorion is increased many fold, just as a tree bybranching is enabled to bear enough leaves to supply it with the necessaryamount of carbon dioxid from the air. When the villi first bud from the basal chorionic membrane they. DEVELOPMENT OF THE OVUM 41 are covered on their exterior 1)}^ two layers of cells, an inner designatedas Langhans layer and an outer designated as syncytium, whose struc-ture is too complicated to be considered here. The core of each villusis made up of connective tissue, which is derived from the originalmesoderm of the egg. Within a few days the rudiments of fetal bloodvessels appear in the connective tissue of the villi and by the end ofthe first month they become coupled up with the circulatory systemof the embryo through the umbilical cord, thus affording a direct pas-sage from the interior of the embryo to the periphery of the chorion,but at no time during pregnancy do they communicate with the maternalblood. When, about the middle of pregnancy, the fetal need for foodhas been greatly increased, the inner or Langhans layer of cells dis-appears, thus removing one of the barriers between the fetal andmaternal bloods and promoting a freer interchange between
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectobstetrics, bookyear1
