FOETAL NUTRITION: THE PLACENTA 425 



foetal placenta " (Chipman ; see also p. 369). By the influence 

 of the trophoblast the decidual cells increase in size and 

 become multinucleate (Maximow 1 ). They lose their peri- 

 vascular position and pervade the whole of the region. In their 

 formation all traces of the gland ducts are lost, the cells of the 

 latter appearing to serve as pabulum for the decidual cells. 

 In the region of the uterine sinuses the blood-vessels dilate 

 to form large spaces, and the decidual cells remain uninucleate 

 till a considerably later period. The junction between the two 

 zones is marked by the blind ends of the glands, which are 

 filled with degenerated epithelium. In section each appears as 

 an island of glandular symplasma. 



At the tenth day the allantois joins the outer wall of the 

 blastocyst over the site of the future placenta. The trophoblast 

 of this region is differentiated into two layers, the plasmodiblast 

 and the underlying cytoblast. The latter disappears before the 

 end of pregnancy. Processes of vascular mesoblast invade the 

 trophoblast at intervals, and break it up into columns. At the 

 same time the fcetal tissues continue to advance and surround 

 maternal capillaries, the endothelium of which they replace. 

 In the zonary placenta of Carnivora the trophoblast surrounds 

 the vessels without destroying the endothelium. In the rabbit 

 the ectodermal processes are hollow tubes which surround the 

 vessels ; they are closed on the foetal side and open on the 

 maternal side. Their cavity is filled with maternal blood, and 

 externally lies the cytoblast and vascular mesoblast. Such are 

 the " villi." Subsequently the arrangement becomes more 

 complex, each hollow column being divided up into a series of 

 hollow tubes parallel to the original column, and each tube in 

 turn forming a series of hollow tubules. At each division the 

 thickness of fcetal tissue between the maternal blood in the 

 axis and the foetal vessels decreases, till finally there is only a 

 film of trophoblast and the vascular wall. At places the 

 trophoblast even is wanting, and the fcetal endothelium alone 

 intervenes between the two blood-streams. 



The endothelium of the maternal capillaries frequently 

 ruptures just before it is overtaken by the ectoderm, and irre- 



1 Maximow, " Zur Kenntnis des feineren Baues der Kaninchen-Placenta." 

 Arch.f. mikr. Anat., vol. li., 1897. See also ibid., vol. Ivi., 1900. 



