368 THE PHYSIOLOGY OF REPRODUCTION 



time, specialised decidual cells, which have the power of de- 

 stroying the rest of the decidual tissue, have been described in 

 the hedgehog, 1 rat, and other animals. But it is now generally 

 accepted that the foetal ectoderm from the earliest stages of 

 pregnancy is able to disintegrate the cells with which it comes 

 in contact, and to absorb the degenerate products. To that 

 part of the foetal epiblast which is thus adapted for the ac- 

 quirement of embryonic nutriment the name of trophoblast has 

 been given by Hubrecht. 



Along with the gradual absorption of the degenerated parts 

 of the decidua, and the great increase in the extent of the 

 serotinal surface as pregnancy advances, there is probably a 

 continued formation of new decidual elements. Pfannenstiel 

 attributes the new formation to the peri- vascular tissue, and 

 Webster to groups of active cells, the " Ersatz-zellen " of Klein, 2 

 found here and there in the mucosa. Whatever their origin is, 

 we may see, even in the shed placenta at full time, well-formed 

 and apparently healthy decidual elements as well as the 

 fibrinous masses containing cellular fragments. 



Within recent years there has been a tendency to belittle 

 the importance of the connective tissue elements of the placenta. 

 This has been largely due to the wider acceptance of the fetal 

 origin of the syncytium, and to the conception of the placenta as 

 a maternal haemorrhage circumscribed by foetal structures. But 

 the same idea has been encouraged by some who look on the 

 syncytium as maternal, and they adduce as evidence the obvious 

 degeneration in the decidua during the greater part of pregnancy. 

 Pfannenstiel maintains that decidual cells are, from the begin- 

 ning, degeneration forms of the connective tissue cells and are of 

 use only as pabulum to be absorbe^ by the ovum. But during 

 the whole of pregnancy, as mentioned above, there exist in the 

 placenta decidual cells which, in their appearance and staining 

 properties, show no resemblance to degenerated cells. From 

 their abundance and great specialisation they have in all likeli- 

 hood definite functions to perform. Their first formation dates 



1 In a later memoir Hubrecht assigns to these cells, the decicluofracts of 

 the hedgehog, an origin from the outer layer of the trophoblast. See 

 foot-note, p. 470. 



2 Klein, " Entwicklung und Ruckbildung d. Decidua," Zeits. f. Qeburtsh. 

 u. Qyndk., vol. xxii. 



