402 THE PHYSIOLOGY OF REPRODUCTION 



varioiis stages of hyaline degeneration, giving rise in part to the 

 layers .of fihrin, 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 thte serotina 

 and reflexa than in the vera (Webster i). Its gradual diminution 

 during pregnancy indicates ■ 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 time, specialised decidual cells, which have the 

 power of destroying the rest of the decidual tissue, have been 

 described in the hedgehog,^ rat, and other animals. But it is now 

 generally accepted that the fcetal 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 acquirement 

 of embryonic nutriment the name of trophohlast has been given by 

 Hubrecht. It forms the outer or ectodermal layer of the false 

 amnion or chorion (see below, p. 412). 



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 perivascular tissue, and Webster to groups of 

 active cells, the " Ersatz-zellen " of Klein,^ 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 foetal origin of 

 the syncytium, and to the conception of the placenta as a maternal 

 haemorrhage circumscribed by fa3tal 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 beginning, degeneration forms of the 



' Webster, Human Placentation, Chicago, 1901. 



^ In a later memoir Hubrecht assigns to these cells, the deciduofracts of the 

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

 p. 496. 



3 Klein, "Entwicklung und Eiickbildung d. Decidua," Zdtsch. f. Gehxirtdi. u. 

 Oynak., vol. xxii. 



