612 THE HUMAN EMBRYO. 



towards the villous zone, is characterised by the presence of patches 

 of the substance spoken of as canalised fibrin, an^ shown by 

 Minot to be produced by a peculiar mode of degeneration of 

 the surface epithelial cells of the chorion. Round the margin of 

 the placenta, the chorionic and decidual layers are so intimately 

 fused that it is impossible to make out a boundary line between 

 them : the decidual cells invade the chorion, and the two layers 

 undergo degenerative changes in common. . 



The outer or decidual layer of the placenta, formed by the 

 decidua serotina (Fig. 254, ds), is about 1-5 mm. thick, and 

 shows the same distinction into inner or compact, and outer or 

 spongy layers already noticed in the decidua vera. Both layers 

 contain very numerous decidual cells, formed, as elsewhere, by 

 modification of connective-tissue cells. The decidual cells of the 

 compact layer are smaller and more crowded than those of the 

 spongy layer. Their actual size varies very greatly, and the 

 largest ones may contain as many as ten nuclei. 



The uterine glands have disappeared from the compact layer 

 of the decidua, but their outer or blind ends persist in the 

 spongy layer as irregular slit-like cavities, filled for the most 

 part with fine granular matter, and retaining in places their 

 lining of glandular epithelium. The outer surface of the spongy 

 layer is closely united with the muscular wall of the uterus ; and 

 groups of decidual cells may penetrate between the muscle fibres 

 at places. 



The middle or villous zone of the placenta is much the 

 thickest of the three, and is also the most important, and the 

 most complicated. It may be described, roughly, as a huge 

 sinus filled with maternal blood, and divided up into a labyrinth 

 of intercommunicating loculi by a framework of fibrous bands 

 and partitions, which run across between the decidual and cho- 

 rionic walls ; the loculi being occupied by forests of arborescent 

 villi, which arise from the chorionic wall and are richly supplied 

 with capillaries derived from the allantoic vessels of the foetus. 



The villi branch extraordinarily freely : their stems arise 

 from the chorion, and the majority of the branches end freely, 

 but many are attached either to the uterine decidua, or to the 

 partitions separating the loculi. 



The foetal blood-vessels. The blood is carried from the foetus 

 to the placenta by the two allantoic arteries, which run in the 



