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Show 102 ON ULASSIFICA'l'ION. membrane in th region of the pla .nta, is traversed mucous ' . by numerous arterial and v nou trunk , w hJCh . an·y. tho blood t d fronl the placenta; ow and the layer of. l c1dua 1nto which the chorionic villi do n t p n trat a qun· a cavernous, or cellular structure from b coming burrow d, as it w ro, by the innnme;able sinuses int which these art rial and venous trunks Fig, ..J:. I . .A lhg. 41.- 'ection of the Human teru · and J>la<' nta at the thirtieth week of pregnancy. (After Ecker. )- A, umbilical c nl ; B, chorion; \ the fa· tal villi separated by process s of; D, cavernous decidua; E, F, •, wall of the uterus. op n. In th pr f I r tujtion, th decidua set·otina splits thr ugh thi cellulo,r lny r, ntl th . ·ur rficial part of it comes away with th \ uu1 ili al c r ], torr th r with th fooial mem· bran and th r t f th clecid1-~a; whil \ th deeper layer, und rg ing ft tty d g n 'n ti n unl r .·olution, is more or less com1 1 t ly brouo-ht {way , ith h loch~·(t, and give place to a new Jnn u 1n rnbr{ 1 , whi ·h i · d lop d throughout the r ·t f th ut ru., dnriu · pr b'lUU <·y; Lut, pos iLly, arises only aft r d liv ry >V r th pl~ c ntal ar ·a. In ih Pig th pla :1uta i · ~ln iufiuitcly ·impler structure. DECIDUA'l'E AND NON-DECIDUATE PLACENTM. 103 No "decidua'' is developed ; the elevations and depressi ns of the unimpregnated uterus simply acquire a greater size and vascularity during pregnancy, and cohere closely with the chorionic villi, which do not become restricted to one spot, but ·are developed from all parts of the chorion, except its poles, and remain persistent in the broad zone thus formed throughout footal life. The cqhesion of the footal and maternal placontoo, however, is overcome by slight maceration or postmortem change; and, at parturition, the footal villi are imply drawn out, like fingers from a glove, no vascular substanee of the 1nother being thrown off. The process by which the mucous membrane of the uterus returns to its unimpregnated condition after parturition in the pig has not been traced. The extreme cases of placentation exhibited by man and by the Pig 1nay be termed, with Von Baer and Eschricht. from the character of the maternal placenta, "caduceus" ancl "non-caducous," or, from the degree of cohesion of the two placentoo in parturition, " coherent " and "incoherent ·" or what perhaps would be better still, the two l\Iammals ' may' be spoken of as "deciduate" and "non-deciduate."* But, whatever terms be employed, the question for the clus ifier is to inquire what mammals correspond with Man and what with the Pig, and whether the groups of deciduate and non~ eciduate Monodelph~a thus formed, are natural groups, or, 1n other words, contain such orders as can be shown, on other grounds, to be affined. With respect to the deciduate Monodelphia, it is certain that ~he apes agree, in the main, with n1an in placental, a in other Important characters; and, so far as has hitherto been observed (though our knowledge of the placentation of the Len1tus is very de.fective), their placentm differ from those of lVf an only in present1ng a more marked lobation-a character which occurs as a variety in ~Ian. * It is, of course, ?Y no moans intended to suggest by these terms, that the homologue of the demdua does not exist in the "non-decicluato" Mammals. The mucous.mc~brauc of tho uteru.s becomes hypcrtrophiou during pregnancy in both t~w dcculun.te and tho non-dcmcluatc Mammals ; but it is thrown off. and · r1sc t o a " d em' d ua " on1 y m· the one of those two groups. • so gives |