1 68 THE EVOLUTION OF MAN. 



membrane is a peculiar modification, afterwards lost, of the 

 superficial layer of the original mucous membrane of the 

 uterus. The placental membrane is that portion of the 

 preceding which is completely modified by the intrusion of 

 the chorion-tuffcs and is employed in forming the placenta. 

 Lastly, the inner deciduous membrane is formed by a 

 ring-shaped fold of the mucous membrane (at the point 

 of union of the d. vera and the d. serotina) which rises, 

 grows round the egg, and closes in the same way as the 

 amnion. 160 



The peculiar anatomical characters which mark the human 

 egg-membrane re-occur, in the same form, only in Apes. All 

 other Discoplacental Animals present greater or less differ- 

 ences in these points, the conditions being generally more 

 simple. This is the case, for instance, in the structure of 

 the placenta itself, in the coalescence of the chorion tufts 

 with the decidua serotina. The matured human placenta 

 is a circular (rarely oval) disc of a soft, spongy character, 

 6 to 8 inches in diameter, about 1 inch thick, and weighing 

 from 1 to 1J lb. Its convex, external surface (that which 

 coalesces with the uterus) is very uneven, and tufted. Its 

 internal, concave surface (that which is turned towards the 

 cavity of the egg) is quite smooth, and clothed by the amnion 

 (Fig. 198, a). From near the centre of the placenta springs 

 the navel cord (funiculus umbilicalis), the development 

 of which we have already observed (vol. i. p. 383). It also is 

 coated by the amnion as with a sheath, which at the nave] 

 end passes directly into the abdominal skin (Fig. 200, 201). 

 The mature navel cord is a cylindrical cord, coiled spirally 

 around its axis, and usually about 20 inches long and J inch 

 thick. It consists of gelatinous connective tissues (" Whar- 



