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PLACE (through Fr. from Lat. platea, street; Gr. irXarw, 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 applications are to an open space in a town, a group of buildings, row of houses, or as the name of a residence or manor-house. In certain cases this latter use together as the embryo enlarges, and then, as pressure continues, atrophy. The allantoic stalk elongates enormously, and in its later stages 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 stallc and the allantois; the latter is an entodermal outgrowth 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, 1908); and, for literature, O. Hertwig's Handbuch 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.

Unchanged layer. Stratum From A. H. Young and A. Robinson, in Cunningham's Ten-Book of Anatomy.

FIG.2. Diagram. Later stage in the development of the placenta, showing the relations of the foetal villi to the placental sinuses, the fusion of the amnion with the inner surface of the chorion, and the thinning of the fused deciduae (capsularis and vera). phylum as Salpa, a placenta is formed, and the embryo is nourished within the body of its parent. In some of the viviparous sharks, e.g. the blue shark (Carcharias), the yolk-sac has ridges which fit into grooves in the wall of the oviduct and allow an interchange of materials between the maternal and foetal blood. This is an example of an " umbilical placenta." In the viviparous blennies (Zoarces viviparus), among the teleostean fishes, two or three hundred young are nourished in the hollow ovary, which develops villi secreting nutritive material. Among the Amphibia the alpine salamander (Salamandra atra) nourishes its young in its oviducts until the gilled stage of development is past, while in the Reptilia the young of a viviparous lizard (Seps chalcides) establish a communication between the yolk-sac anteriorly and the allantois posteriorly, on the one hand, and the walls of the oviduct on the other. In this way both an umbilical and an allantoic placenta are formed.

The mammals are divided into Placentalia and Aplacentalia; in the latter group, to which the monotremes and most marsupials 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 yark (Orycteropus), there is a " zonary -placenta " which forms a girdle round the embryo. In sloths and lemurs the placenta is domeshaped, 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 are simply withdrawn, the decidua being 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. Turner, Lectures on the Comparative Anatomy of the Placenta (Edinburgh, 1876) ; A.Robinson, " Mammalian Ova and the Formation of the Placenta," Journ. Anal, 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.)

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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