452 THE. PHYSIOLOGY OF REPRODUCTION 



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. 126). Each fold is divided by transverse fissures into rectangular 

 areas, the coussinets of Hollard. 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 (Bischoff 1 ), which is thus bi-lobed (Fig. 127). 

 The folds of the mucosa are essentially increased areas of the nmcosal 

 connective tissue, but they differ from the cotyledons of Ruminants 



in having glands. 



On the entrance of a fertilised 

 ovum into the uterus, the folds, 

 especially the ob-placental, become 

 shortened, and a localised actual 

 cavity appears which is occupied 

 by the blastocyst. At the same 

 time there is a marked hyperplasia 

 of the cellular connective tissue of 

 the placental and peri-placental 

 folds, leading to a thickening of 

 their bases (Chipman 2 ). By the 

 sixth day the capillaries are also 

 increased in these regions. In the 

 ob-placental folds appear enormous 

 giant-cells, derived by a process of 

 " degenerative hypertrophy " from 

 the epithelium of the surface and 

 glands. 3 They persist till at least 

 mid-pregnancy, and are probably 



absorbed by the trophoblast overlying the yolk-sac: In the placental 

 lobes the epithelial cells proliferate and fill up the superficial culs-de- 

 sac of the mucous membrane. The glands are as yet unchanged, and 

 the increased blood supply leads to a free secretion which is usually 

 considered to be added to the albumen -layer, and then to be absorbed 

 by the trophoblast. There is no appreciable transudation of lymph 

 such as occurs in Ruminants. 



1 Bischoff, Entwickelung des Kaninchen-Eies, Braunschweig, 1842. 



2 Chipman, " Observations on the Placenta of the Rabbit, with Special 

 Reference to the Presence of Glycogen, Fat, and Iron," Lab. Rep., Roy. 

 Coll. Phys., Edinburgh, vol. viii., 1903. The development of the placenta is 

 carefully traced in a complete age-series of pregnant rabbits and admirably 

 figured by many photo-micrographs. The account as given here is based 

 mainly on Chipman's monograph, but the phraseology is sometimes changed. 



3 Mr Hammond suggests that these may be detached cells from the tropho- 



FIG 127. Transverse section of a 

 seven days' gestation sac of the 

 rabbit (Chipman). The placental 

 folds (coussinets) are large (a) : 

 the muscular walls of the sac 

 are thin. 



blast. Cf. Hammond, "On the Causes Responsible for the Developmental Progress 

 of the Mammary Glands in the Rabbit," Proc. Roy. Soc., B., vol. 



Ixxxix., 1917. 



