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Encyclopedia Britannica - Main :: PIG-POL |
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PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide) , a definite position in space, whether of limited or unlimited extent, situation or locality; also position in a series or rank; or an office, or employment, particularly one in the service of a government. Special
house
Cavity which becomes coelom. Fibrin plug. Decidua vera. Unchanged layer. Stratum spongiosum. Uterine Stratum compactum. mimosa. Ectodermal villus enclosing space containing maternal blood. From A. H. Young and A. Robinson, in Cunningham's Text-Book of Anatomy. It is the decidua basalis which is specially interesting in considering the formation of the placenta
chorionic villi." Later, the " allantoic " or " abdominal stalk " grows from the mesoderm of the 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 original
tissue , known as syncyrium, derived from the chorionic epithelium, so that the embryo is able to take its supply of oxygen and materials for growth from the blood of its 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
together as the embryo enlarges, and then, as pressure continues, atrophy. The allantoic stalk elongates enormously, and in its later sages contains two 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-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 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 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 form the placenta.For further details see Quain's Anatomy, vol. i. (London, 19o8); and, for literature, O. Hertwig's ilandbuch der Entwickelungslehre (Jena). Comparative Anatomy.If the placenta is to be regarded as a close union between the vascular system of the parent and embryo, the 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 great deal of yolk, and the young, 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 time, as in the case of the hedgehog, the bandicoot and the mouse. In most of the lower placental mammals the allantois is much more 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 cornu of the uterus. This is known as a " diffuse placenta," and is met with in the pangolin, pig, hippopotamus, camel, chevrotain, horse, rhinoceros, tapir and whale. When the villi are collected into a number of 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 giraffe. In the Carnivora, elephant, procavia (Hyrax) and aard vark (Orycteropus), there is a " zonary-placenta " which forms a 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 ant-eater, 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 birth
For further details see S. W. W. Turner, Lectures on the Comparative Anatomy of the Placenta (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. Parker (London, 1907). (F. G. P.) End of Article: PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide) If you wish, you can link directly to this article.
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