See also:rank; or an
See also:office, or employment, particularly one in the service of a
See also:government .
See also:Special applications are to an open space in a
See also:town, a
See also:group of buildings,
See also:row of houses, or as the name of a residence or
See also:house . In certain cases this latter use Cavity which becomes coelom .
See also:Fibrin plug . Decidua
See also:vera . Unchanged layer . Stratum spongiosum . Uterine Stratum compactum.
See also:mimosa . Ectodermal villus enclosing space containing maternal
See also:blood . From A . H .
See also:Young and A .
See also:Robinson, in
See also:Cunningham's Text-
See also:Book of Anatomy . It is the decidua basalis which is specially interesting in considering the formation of the
See also:placenta . That
See also:part which is nearest the ovum is called the " stratum compactum," but farther away the uterine glands dilate and give a spongy appearance to the mucous membrane which earns this particular layer the name of " stratum spongiosum." Processes grow out from the
See also:surface of the ovum which penetrate the stratum compactum of the decidua basalis and capsularis and push their way into the enlarged maternal blood sinuses; these are the chorionic villi." Later, the " allantoic " or " abdominal stalk " grows from the mesoderm of the
See also:hind end of the embryo into the chorionic villi which enter the decidua basalis, and in this blood-vessels pass which push their way into the maternal blood sinuses . Eventually the
See also:original walls of these sinuses, together with the false amnion, disappear, and nothing now separates the maternal from the foetal blood except the delicate walls of the foetal vessels covered by some nucleated noncellular tissue, known as syncyrium, derived from the chorionic epithelium, so that the embryo is able to take its supply of
See also:oxygen and materials for growth from the blood of its
See also:mother and to give up carbonic acid and excretory matters . It is the gradual enlargement of the chorionic villi in the decidua basalis together with the intervillous maternal blood sinuses that forms the placenta ; the decidua capsularis and vera eventually become pressed together as the embryo enlarges, and then, as pressure continues, atrophy . The allantoic stalk elongates enormously, and in its later sages contains two
See also:arteries (umbilical) and only one vein (owing to the obliteration of the right one) embedded in some loose connective tissue known as " Wharton's jelly." At first the stalk of the yolk-
See also:sac is quite distinct from this, but later the two structures become bound up together (see fig . 2), after which they are known as the " umbilical
See also:cord." A distinction must be made between the allantoic stalk and the allantois; the latter is an entodermal out-growth from the hind end of the mesodaeum or
See also:primitive alimentary canal, which in the human subject only reaches a little way toward the placenta . The allantoic stalk is the mass of mesoderm containing blood-vessels which is pushed in front of the allantois and, as has been shown, reaches and blends with the decidua basalis to
See also:form the placenta . For further details see
See also:Quain's Anatomy, vol. i . (
See also:London, 19o8); and, for literature, O . Hertwig's ilandbuch der Entwickelungslehre (
See also:Jena) .
See also:Comparative Anatomy.—If the placenta is to be regarded as a close union between the vascular
See also:system of the
See also:parent and embryo, the
See also:condition may be found casually scattered throughout the phylum of the Chordata .
In such a very lowly member of the Placenta.belong, the ova have a
See also:deal of yolk, and the young,
See also:born in a very immature condition, finish their development in their mother's pouch; but although these mammals have no allantoic placenta there is an intimate connexion between the walls of the yolk- sac and the uterine mucous membrane, and so an umbilical or omphalic placenta exists . The name Aplacentalia therefore only means that they have no allantoic placenta . Among the Placentalia the umbilical and allantoic placentae sometimes coexist for some
See also:time, as in the case of the
See also:hedgehog, the
See also:bandicoot and the
See also:mouse . In most of the
See also:lower placental mammals the allantois is much more
See also:developed than in man, and the most primitive type of placenta is that in which villi are formed over the whole surface of the chorion projecting into the decidua of the tubular
See also:cornu of the uterus . This is known as a " diffuse placenta," and is met with in the
See also:pig, hippopotamus, camel,
See also:tapir and
See also:whale . When the villi are collected into a number of
See also:round tufts or cotyledons, as in most ruminants, the type is spoken of as a " cotyledonous placenta," and an intermediate stage between this and the last is found in the
See also:giraffe . In the
See also:elephant, procavia (Hyrax) and aard vark (Orycteropus), there is a " zonary-placenta " which forms a
See also:girdle round the embryo . In sloths and lemurs the placenta is dome-shaped, while in rodents, insectivores and bats, it is a ventral disk or closely applied pair of disks, thus differing from the dorsal disk of the
See also:armadillo and higher Primates, which is known as a " metadiscoidal placenta." It will thus be seen that the form of the placenta is not an altogether trustworthy indication of the systemic position of its owner . In the diffuse and cotyledonous placentae the villi do not penetrate very deeply into the decidua, and at
See also:birth are simply withdrawn, the decidua being
See also:left behind in the uterus, so that these placentae are spoken of as non-deciduate while other kinds are deciduate . For further details see S . W . W .
See also:Turner, Lectures on the Comparative Anatomy of the Placenta (
See also:Edinburgh, 1876) ; A.Robinson, " Mammalian Ova and the Formation of the Placenta," Journ . Anat. and Phys . (1904) xxxviii., 186, 325 . For literature up to 1906, R . Wiedersheim's Comparative Anatomy of Vertebrates, translated and adapted by W . N .
See also:Parker (London, 1907) . (F . G .
PLACARD (15th cent. Fr. plackart, from plaquier; mo...
PLACENTA (Lat. for a cake)
There are no comments yet for this article.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.
Links to articles and home page are encouraged.