FCETAL NUTRITION: THE PLACENTA 421 



seventh day, when the blastocyst is about five milHmetres 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 speciahsed 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 : ^ 



Fig. 100. — Transverse section of a four days' gestation sac of the rabbit. 

 The mucosa is differentiated into six definite folds. The two folds 

 nearest the mesometrium are the largest and mark the site of placental 

 attachment. (From Ohipman's "The Placenta of the Rabbit," Labor. 

 Rep., Boy. Coll. Physic, Edinburgh, vol. viii., 1903.) 



p, p', placental folds ; n, n! , peri-placental folds ; o, o', ob-placental folds. 



placental folds, the largest, situated one on each side of the 

 groove corresponding to the insertion of the mesometrium ; 

 ob-placental folds, the smallest, opposite the mesometrium ; 

 peri-placental folds, intermediate in position and size (Fig. 100). 

 Each fold is divided by transverse fissures into rectangular 

 areas, the coussinets of HoUard. At the onset of pregnancy two 

 of these areas on the placental folds, placed one on either side 

 of the mesometrial groove, hypertrophy and form the maternal 

 part of the future discoid placenta (Bischofi*), which is thus 



' See Hertwig's Entvnckhingsgeschichte des Menschen und der Wirbelthiere, 

 1906. 



^ HoUard, "Recherche sur le Placenta des Rongeurs," Annales dea 

 Sciences Naturelles, 1863. 



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



" Bischoff, Entwickelung dea Kaninchen-Eies, Braunschweig, 1842. 



