XI.] THE PLACENTA. 351 



of the yolk-sac is becoming narrow, a somatic stalk 

 connecting the amnion with the walls of the embryo is 

 also formed, and closely envelopes the stalk both of the 

 allantois and the yolk-sac. The somatic stalk together 

 with its contents is known as the umbilical cord. The 

 mesoblast of the somatopleuric layer of the cord de- 

 velops into a kind of gelatinous tissue which cements 

 together the whole of the contents. The allantoic ar- 

 teries iu the cord wind in a spiral manner round the 

 allantoic vein. The yolk-sac in many cases atrophies 

 completely before the close of intra-uterine life, but in 

 other cases it, like the other embryonic membranes, is 

 not removed till birth. The iutra-embryonic portion of 

 the allantoic stalk gives rise to two structures, viz. to 

 (1) the urinary bladder formed by a dilatation of its 

 proximal extremity, and to (2) a cord known as the 

 urachus connecting the bladder with the wall of the 

 body at the umbilicus. The urachus, in cases where 

 the cavity of the allantois persists tUl birth, remains as 

 an open passage connecting the intra- and extra-em- 

 bryonic parts of the allantois. In other cases it gradually 

 closes, and becomes nearly solid before birth, though a 

 delicate but interrupted lumen would appear to persist 

 in it. It eventually gives rise to the ligamentum vesicae 

 medium. 



At birth the fcetal membranes, including the festal 

 portion of the placenta, are shed ; but in many forms 

 the interlocking of the foetal villi with the uterine 

 crypts is so close that the uterine mucous membrane is 

 canned away with the fcetal part of the placenta. It 

 thus comes about that in some placentae the maternal 

 and foetal parts simply separate from each other at birth, 



