F(ETAL NUTRITION: THE PLACENTA 367 



of the mucosa has suggested a stimulus for their formation 

 arising from the product of conception. 1 The study of early 

 human specimens has effectually disproved Ercolani's 2 idea 

 that the uterine mucosa was first entirely destroyed by the 

 developing ovum, and then replaced by decidual tissue formedr 

 from the cells of the vessel walls. Such an endothelial pro- 

 liferation does, however, occur in certain animals, e.g. hedgehog 

 (Hubrecht 3 ) and bat (Nolf 4 ), and probably in ectopic gestation 

 in Man. 



The rapid increase in the size and number of the decidual 

 cells, together with the dilatation of the blood-vessels, leads to 

 a great increase in the thickness of the serotina. At a certain 

 stage it reaches its full development, and then gradually 

 diminishes till, at the end of gestation, it forms only a thin 

 layer, and even disappears entirely at parts so that the villi 

 impinge on the muscular wall. 



Individual decidual cells have probably a short life-history. 

 Even at a comparatively early period many of them are found 

 in various stages of hyaline degeneration, giving rise in part to 

 the layers of fibrin, and as pregnancy advances there is a gradual 

 extension of the fibrinous change. The degeneration of the 

 decidual tissue would seem to be due to the influence of the 

 foetal epiblast, as in Man it occurs much earlier and more 

 abundantly in the serotina and reflexa than in the vera 

 (Webster 5 ). Its gradual diminution during pregnancy in- 

 dicates an absorption of the decidua. That maternal tissues 

 do not play a large part in this absorption is probable from the 

 small number of leucocytes and the absence of lymph-channels 

 in the neighbourhood of the fibrinous masses. At the same 



1 Under abnormal conditions the formation of decidual cells occurs even 

 although no ovum is present in the uterus, e.g. in tubal pregnancy in the 

 human female. Whether this indicates a chemical stimulus from the ovum, 

 or perhaps from the corpus luteum, effected through the blood-stream, is not 

 yet known (see p. 491). 



2 Ercolani, " Sulla unita del tipo anatomico della placenta," Mem. delV 

 Accad. di Bologna, 1876. 



3 Hubrecht, "The Placentation of Erinaceus europceus," Quar. Jour. 

 Micr. Science, vol. xxx., 1889. 



4 Nolf, " Modifications de la muqueuse uterine pendant la gestation chez 

 le murin," Arch, de Biol., vol. xiv., 1896. 



6 Webster, Human Placentation, Chicago, 1901. 



