FCETAL NUTRITION: THE PLACENTA 



451 



of the placental area the glands disappear early, and a great increase 

 in the interglandular stroma occurs, as in Eodents.^ 



EODENTIA. — Among" the Eodents there are variations in the mode 

 of attachment. It is centric in the rabbit, excentric in the mouse 

 and rat, and interstitial in the guinea-pig. In all the ultimate form 

 of the placenta is discoid. 



It was in Eodents that the proliferation and vascularisation of 

 the trophoblast were first described by Selenka.^ Later Duval* 

 gave a fuller account of the earlier stages, and Hubrecht dis- 

 covered the same conditions in other 

 orders. 



Babbit. — The fertilised ovum of 

 the rabbit, clothed by the prochorion, 

 reaches the uterus at the beginning 

 of the fourth day after coitus. At 

 first it has no fixed position ; but by 

 the seventh day, when the blastocyst 

 is about five millimetres in diameter, 

 the prochorion lies so closely on the 

 surface of the uterus that it fixes 

 the ovum. At the end of the eighth 

 day the . prochorion ruptures, and 

 the blastodermic vesicle probably 

 collapses at the same time by injury 

 to its wall.* 



The " mature " uterine mucous 

 membrane of the non-pregnant 

 rabbit already shows specialised 



FiG._ 126. — Transverse section of a 

 four days' gestation sac of the 

 rabbit. T^he mucosa is differ- 

 entiated into six definite folds. 

 The two folds nearest the 

 mesometrium are the largest 

 and mark the site of placental 

 attachment. (From Chipman's 

 "The Placenta of the Rabbit," 

 Labor. Rep., Roy. Coll. Phys., 

 Edinburgh, vol. viii., 1903.) 



p, p', Placental folds ; n, «.', 

 peri-placental folds ; 0, 0', ob- 

 placental folds. 



structures, which are of importance 

 for the attachment and nutrition of a future embryo. These consist 

 of symmetrical pairs of longitudinal folds, first described by HoUard,* 

 and subsequently named by Minot:* placental folds, the largest. 



> Hubrecht (Quar. Joiir. Micr. Science, 1908) draws attention to the peculiar 

 position of Hyrax. It has many archaic peculiarities, and has been placed 

 near Eodents, elephants, and Ungulates by diflferent authors. Yet its placental 

 characters resemble those of the hedgehog and man. This he takes as strong 

 evidence that the type of placenta found in Hyrax, the hedgehog, and man 

 diverges less widely from the primitive type than the placenta of Ungulates 

 and Eodents. 



^ Selenka, Keimhldtter und Primitivorgane der Maus, 1883. 



' Duval, "Le Placenta des Eongeurs," Jour, de I'Anat. et de la Phys., 1889-92. 



* See Hertwig's EntwiakLungsgeschiGhte des McTischen und der Wirbelthiere, 

 1906. 



^ Hollard, " Eecherehe sur le Placenta des Eongeurs,'' Annales des Sciencen 

 Xatmelles, 1863. 



' Minot, "Die Placenta des Kaninchens," Biol. Centralhl., vol. x., 1890. 



