FCETAL NUTRITION: THE PLACENTA 395 



(Fig. 87), radiating from small circular spots, twenty or thirty 

 to the square inch, which represent the gland-mouths (Turner 1 ). 

 It is usually stated that the uterine surface epithelium remains 

 intact ; but Assheton has recently proved that it shows signs of 

 degeneration as early as the eighth day, and at the eighteenth 

 day is reduced to a thin layer. Three days later, however, it 

 again appears normal and is formed of long columnar cells, to 

 the ends of which the trophoblast fits closely, sending proto- 



Blv 



FIG. 88. Section through the wall of the uterus and the blastocyst of the 

 pig at the twentieth day of pregnancy (Assheton). 



mes, mesoblast ; Bl.v, fnjtal vessel ; Tr, trophoblast ; Ep, long columnar 

 epithelium of uterine surface. 



plasmic processes between the cells (Fig. 88). These processes 

 may even reach past the epithelium to the underlying layer 

 of dilated capillaries (Robinson), and absorb nutritive material 

 or effect gaseous exchanges. 



The trophoblast is single-layered throughout, and, after the 

 first three weeks, forms a syncytium. Internally to it lies the 

 mesoblast, which in the main follows its ridges and furrows, 

 but occasionally bridges across a fold. It is vascularised by the 

 vessels of the allantois, which completely surrounds the embryo. 

 The gland-mouths lie along the course of the vessels (Assheton). 

 No formation of villi takes place, and the attachment never 



1 Turner, Lectures on the Comparative Anatomy of the Placenta, Edinburgh, 

 1876. 



