FORMATION AND STRUCTURE OF THE PLACENTA. 



53 



of the two fluids, nor is it possible to inject the foetus from the mother through the 

 placenta, or the mother from the foetus. 



The question, which, has been more than once raised, whether the intervillous spaces 

 normally contain blood, may be now regarded as settled in the affirmative. It has been 

 usually held that they represent capillaries or veins of the decidua, which have become 

 dilated and fused together to such an extent as to occupy the whole thickness of the 

 placental part of that membrane, with atrophy of the intervening decidual tissue, which 

 merely remains as a covering to the villi. But since the placental sinuses appear to be 

 bounded superficially by the chorionic covering of the ovum, and it is in most places not 

 possible to detect any decidual tissue between them and the chorion, it has been conjectured by 

 some writers (Kolliker, Langhans) that they have become formed by extravasation of blood into 

 a space between the ovum and the decidua, into and across which space the chorionic villi 

 have grown. Although the development of these structures is insufficiently known in Primates, 

 it has been shown in various mammals (Selenka, Duval, Hubrecht, Masius) that the first 

 attachment of the blastodermic vesicle to the uterine wall is effected by the external layer of 

 the epiblast, which sometimes splits off over the embryonic area as a distinct layer, and 

 which, in some animals (e.g., hedgehog), becomes greatly thickened, and is connected by 

 epiblastic villi to the decidua. This external layer of epiblast, for which Hubrecht has pro- 

 posed the general name of trophoblast, causes the absorption of the uterine epithelium both 

 of the surface and of the glands (where this epithelium has not previously been cast off) 

 and comes directly in contact with the enlarging decidual vessels, the endothelium of which 

 is actively proliferating. Within the thickened trophoblast clefts now make their appear- 

 ance and are presently found to be occupied by maternal blood, which is derived from the 

 vessels of the adjacent hypertrophied decidua. This blood flows therefore into spaces in the 

 trophoblast, which are only bounded by foetal epiblast, and ^B primary placental circulation 

 may be formed before any foetal blood-vessels have reached the chorion. Subsequently, when 

 the vascular mesoblastic villi become formed they extend into these spaces, pushing before 

 them the epiblast ; by this layer they remain permanently covered and it also lines the 

 enlarged spaces into which they have extended. 



The placenta is composed of two parts, one foetal, composed of chorion with 

 its villi ; the other maternal, formed from decidua serotina. In its completely 



Fig. 57. TRANSVERSE SECTION OF A VILLTJS FROM 



A PLACENTA OF SEVEN MONTHS (Minot). 



Three blood-vessels are seen within the villus, 

 imbedded in a jelly-like connective tissue containing 

 cells and fibres ; a, a, cell-layer covering villus 

 (epiblast according to Minot ; according to others 

 of decidual origin) ; /, a thickened portion of this 

 cell -layer, which has undergone a fibrinous transfor- 

 mation (canalised fibrin). 



developed condition, it is a circular discoid 

 mass, weighing about a pound, 7 or 8 inches 

 in diameter, thickest at the centre (1^ inch), 

 and thinning away towards the edges, 

 which are continuous with the compara- 

 tively thin coalesced deciduas and chorion. 

 Its inner surface is smooth and concave, 

 and is closely covered by the amnion as by 



a serous membrane ; under this the larger branches of the umbilical vessels course 

 before dipping into the substance of the placenta. From near the centre of the 

 organ the umbilical cord passes off to the foetus. Its outer surface is incorporated 

 with the uterine wall, but when detached from this by tearing through the spongy 

 tissue of which the deeper part of the decidua is formed (as occurs in parturition at 

 the expulsion of the foetus), the outer surface appears ragged and irregular, in 

 striking contrast to the smooth amnion- covered inner surface. Examined under 

 the microscope, the chorionic tissue (villi) of the placenta is found to be composed 

 of jelly-like connective tissue, with branched and anastomosing cells (fig. 57) ; in 

 gome parts of the larger stems white fibres are seen. What remains of the decidual 



