THE PLACENTA. 441 



intervening between them, until ultimately the two sets of 

 blood-vessels are separated by exceedingly thin partitions. 



The successive steps in this process are as follows ; Duval's 

 descriptions being mainly followed in the account here given. 



From the tenth day onwards the growth of the vascular 

 septa, or villi, from the allantois into the ectoplacenta proceeds 

 very rapidly, so that the latter becomes cut up into a series 

 of radially arranged columns, or lobules, within which lie the 

 lacunas opening into the maternal capillaries. At this stage 

 the foetal blood is separated from the maternal blood by three 

 structures : (i) the endothelial wall of the fcetal or allantoic 

 capillaries ; (ii) a layer, several cells in thickness, of the ecto- 

 placental epithelium ; (iii) the endothelial wall of the maternal 

 capillaries. There is some doubt, however, with regard to the 

 third layer ; according to Duval, this has already disappeared, 

 and the maternal vessels of the placenta are merely lacunar 

 spaces hollowed out in the ectoplacental epithelium, and devoid 

 of true walls. 



During the twelfth to the fourteenth days, each of the ecto- 

 placental columns or lobules becomes subdivided, by longitudinal 

 folding of its walls, and ingrowth of septa, into a set of closely 

 placed parallel tubules, the general direction of which is radial, 

 i.e. vertical to the inner surface of the uterus. 



These lobules, in the later stages, become larger and more 

 minutely subdivided, and by the nineteenth or twentieth day the 

 relations are as shown in Fig. 170. Each of the two placental 

 lobes now consists of a number of lobules, PH, which are some- 

 what fusiform in shape, radially arranged, and packed closely 

 together side by side. Each lobule is further subdivided into 

 a complicated system of branching tubular passages, which at 

 each end of the lobule open into larger chambers, UP. Through 

 these passages, which, according to Duval, are simply lacunas 

 excavated in the ectoplacental epithelium, the maternal blood 

 circulates. Large afferent channels, derived from the uterine 

 arteries, convey the maternal blood directly to the dilated 

 chambers at the inner ends of the lobules, next to the surface of 

 the uterus. From these chambers it flows back, through the 

 complicated system of tubules of which the lobule consists, to 

 .the chambers at the outer ends of the lobules, from which it is 



