TFIE PLACENTATION OF PERAMELES. 409 



larctus, says (12, p. 657), " The wliole vascular area is covered 

 by Hat cells of the subzonal membrane." In a uterine embryo 

 of Macropus ruficollis at a stage slightly later than the one 

 of Perameles under consideration, 1 find that the ectoderm of 

 the vascular omphalopleure consists of a fairly thick flattened 

 layer, which more nearly resembles the entoilerni of the vas- 

 cular omphalopleure of Perameles than the ectoderm of the 

 same. 



So far, then, as the forms mentioned are concerned, it appears 

 that the extreme thinness of the ectoderm of the vascular 

 omphalopleure in Perameles is an exceptional and noteworthy 

 feature. A glance at fig. 6, which includes the whole of the 

 extent of the vascular omphalopleure on one side, will show 

 that, although the latter is separated in sections from the 

 syncytium, there is an accurate and fairly close correspond- 

 ence between the two, elevations in the one corresponding to 

 depressions in the other, and vice versa. The vascular 

 om[)halopleure, in other words, appears moulded to fit the 

 irregular surface of the maternal syncytium, and without 

 doubt during life the two surfaces were accurately apposed 

 the one to the other, the above-mentioned projecting ectoderm 

 cells even serving as an actual attachment. It has already 

 been pointed out that the portion of the syncytium in relation 

 to the vascular omphalopleure possesses a very rich network 

 of maternal capillaries, on and just beneath its surface, and 

 that at this stage it is even relatively much more vascular than 

 the allantoic placental syncytium. The foetal capillaries of 

 the vascular area are thus only separated from the maternal 

 by a thin and delicate ectodermal layer, plus a thin layer of 

 syncytial protoplasm, the latter often absent indeed where the 

 maternal capillaries actually reach the surface. 



These facts point to the conclusion that, prior to the period 

 of functional activity of the allantoic placenta, the placental 

 function is subserved by the close contact of the vascular 

 omphalopleure with the vascular maternal syncytium, an ar- 

 rangement which we are therefore justified in designating as 

 an actual yolk-sac placenta. 



