FCETAL NUTRITION: THE PLACENTA 455 



of unimbcleate desidual cells. " It suggests a reaction of the matdrnal 

 placenta to the 'parasitic' foetal placenta" (Chipman; see also p. 403). 

 By the influence of the trophoblast the decidual cells increase in size 

 and become multinucleate (Maximow ^). They lose their perivascular 

 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 blasto- 

 cyst 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 foetal 

 tissue between the maternal blood in the axis and the fcetal vessels 

 decreases, till finally there is only a film of trophoblast and the 

 vascular wall. At places the trophoblast even is wanting, and the 

 foetal endothehum alone intervenes between the two blood-streams. 



The endothehum of the maternal capillaries frequently ruptures 

 just before it is overtaken by the ectoderm, and irregular blood 

 extravasations are formed, and later surrounded by trophoblast. In 

 the deeper layers of the intermediary region, according to Chipman, 

 capillary haemorrhages occur more slowly, and give rise to a fibrinous 

 tissue with red and white blood corpuscles scattered through it. 

 This is similar to the hsematogenous symplasma of Bonnet. It 

 gradually increases in amount, and extends to the region ^of the 

 uterine sinuses. 



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

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



