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Rh together as the embryo enlarges, and then, as pressure continues, atrophy. The allantoic stalk elongates enormously, and in its later stages contains to 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 ecome 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 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. Herwig's Handbuch der Entwicklungslehre (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

From A. H Young and A Robinson, in Cunningham's Text-Book of Anatomy.

2—Diagram. Later stage in the development of the place showing the relations of the foetal villi to the placental sinuses, fusion of the amnion with the inner surface of the chorion, and 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 viviparolls bennies (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 Amphihia 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 placenta 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 ls 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 are simpiy withdrawn, the decidua being left behind in the uterus, so that these placenta 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. ''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).

PLAGIARISM, an appropriation or copying from the work of another, in literature or art, and the passing off of the same as original or without acknowledgment of the real authorship or source. The Lat. plagiarius meant a kidnapper, stealer or abductor of a slave or child though it is also used in the modern sense of a literary pilferer or purloiner by Martial (I. 53, 9). The word plagium is used in the Digest of the offence of kidnapping or abduction, and the ultimate source is probably to be found in plaga, net, snare, trap, cognate with Gr. πλέκειν, to weave, plait. The idea of plagiarism as a wrong is comparatively modern, and has grown up with the increasing sense of property in works of the intellect. (See .)

 PLAGIOCLASE, an important group of rock-forming minerals, constituting an isomorphous series between albite, or soda-felspar and anorthite, or lime-felspar. Intermediate members are thus soda-lime-felspars, which in their crystallographical, optical and other physical characters vary progressively with the chemical composition between the two extremes albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). This variation is continuous in the series but specific names are applied to members falling between certain arbitrary limits, viz: Albite, Ab (= NaAlSi3O8); the Oligoclase, Ab6An1 to Ab3An1; Andesine, Ab3An1 to Ab1An1; Labradorite, Ab1An1 to Ab1An3; Bytownite, Ab1An3 to Ab1An6, Anorthite, An (= CaAl2Si2Os).

All the members of the series crystallize in the anorthic (triclinic) system. They possess a perfect cleavage parallel to the basal pinacoid P (001) and a somewhat less pronounced cleavage parallel to the pinacoid M (010). The angle between these two cleavages varies from 86° 24' in albite to 85° 50' in anorthite. It was on account of the oblique angle between the cleavages that A. Breithaupt in 1847 gave the name plagioclase (Gr. πλάγιος, oblique, and κλᾶν, to cleave) to these felspars, to distinguish them from the orthoclase felspar in which the corresponding cleavage angle is a right angle. It should be noted that the potash—and potash-soda-felspars, (q.v.) an anorthoclase, though also anorthic, are not included in the plagioclase series of soda-lime-felspars. Crystals are