526 MEMBRANA DECIDUA AND PLACENTA. 



Formation of represented as being a metamorphosis of the mucous mem- 

 membrana de- brane itself. Meantime the ovum is itself coated over with a 

 corresponding membrane, designated membrana reflexa, be- 

 cause it was believed by Mr. Hunter to originate in the circumstance 

 that, when the ovum reached the uterine mouth of the Fallopian tube, it 

 there encountered the proper membrana decidua, and, not perforating it, 

 but bearing it onward, gathered a fold, covering, or envelope, which, 

 from its having thus been formed by a reflexion, was appropriately des- 

 ignated by the term specified. It is, however, now admitted that this 

 description of the formation of the membrana reflexa is erroneous, for in 

 reality the ovum is at no time on the outside of the mucous membrane, 

 which is continuous from the cavity of the uterus through the Fallopian 

 tube. The following, therefore, seems to be the more correct description. 

 The presence of the ovum gives rise to an increased development of cells, 

 which rapidly spread around it, and coat it all over, their points of origin 

 being those portions of the uterine mucous membrane with which the 

 ovum is in contact. In this way it receives its deciduous envelope, 

 which, participating duly in its growth, is at the end of the third month 

 in contact with the uterine decidua all over. 



At the stage we are now considering, the nutrition of the embryo is 

 conducted in a special but very temporary way. The yolk of the ovum 

 has no stock of food to maintain the nutritive processes beyond the brief 

 space which transpires in the passage through the Fallopian tube. The 

 duty of nutrition is at this moment assumed by the villous coat of the 

 chorion, which absorbs fluid exuding from the uterine decidua very 

 much after the manner of the spongioles of a plant ; but almost imme- 

 diately the necessity arises of diverting more directly the albumenoid 

 material to the quickly-growing embryo from the yolk-bag, to which it 

 would have gone, and this new destination implies the introduction of 

 new channels of transport, which, under the form of a vascular appara- 

 tus, are now provided. 



About the close of the second month, a proper vascular apparatus for 

 the combined purposes of nutrition, secretion, and respiration 



The placenta. , '. . , , _ . f . . 



makes its appearance: it is the placenta. Its origin is in 

 the little blood-tubes which form in the tufts of the chorion, in man at 

 one point, in ruminants simultaneously at several, giving rise in the for- 

 mer case to one organ, the placenta, as has been said, in the other to 

 many such, or, at all events, to one of a composite structure, the cotyle- 

 dons. The foetal vessels thus arising in the villi of the chorion become 

 intermingled with vessels contemporaneously arising from the uterus ; 

 and though, in some cases, this intermingling is less complicated, so that 

 the maternal and fostal portions are separable, in man the internetting is 

 complete, the principle being to bring the foetal vascular tufts in such a 



