369 



equatorial portion of the bilaniiuar omphalopleure consists of a layer 

 of large cubical cells, while that over the lower polar area is a quite 

 thin layer of mostly flattened cells. There is at this stage no attach- 

 ment to the uterine surface ; the yolk-sac wall merely follows closely 

 the contour of the latter. 



In the next stage the equatorial placental portion of the bi- 

 laminar omphalopleure is found to be definitely attached to the uterine 

 epithelium by means of pseudopodia-like processes arising usually 

 one from each of the now considerably larger and truly trophoblastic 

 ectoderm cells. These processes do not simply fit in between the 

 uterine epithelial cells but actually pass right through the same 

 engulfing the parts of the cells occurring in their course^). In this 

 way they reach the subepithelial tissue of the uterus, in which there 

 are present immediately beneath the epithelium numbers of maternal 

 capillaries. 



The uterine epithelium is now no longer continuous but is 

 broken up into segments separated by the processes of the ecto- 

 derm cells. 



The further changes which supervene may shortly be described 

 as folloM's. The trophoblastic ectodermal cells with their processes 

 gradually increase in size and the latter not only more completely 

 surround the isolated segments of the uterine epithelium but they 

 also grow round and enclose the maternal capillaries underlying the 

 epithelium . 



The separated portions of the epithelium soon begin to degene- 

 rate and eventually form clear slightly staining nests of cells with 

 scanty protoplasm and nuclei almost destitute of chromatin, which 

 superficially resemble the nuclear nests in the syncytium of the 

 pregnant uterus of Perameles. 



The cell outlines between foetal trophoblastic ectoderm cells 

 very soon disappear with the consequent conversion of the layer 

 into a syncytium. The entoderm in protoplasmic continuity with the 



1) Caldwell in his paper on the "Embryonic Membranes in Marsupial 

 Animals" with especial reference to Phascolarctus and M. rufi- 

 11 is describes for these forms a remarkably similar but apparently 

 temporary attachment of the bilaminar omphalopleure to the uterine 

 surface. He says "they [the ectodermal cells of the omphalopleure just 

 outside the sinus terminalis] throw out pseudopodia-like processes, which 

 fit in between the cells of the uterine epithelium and serve to attach 

 the blastodermic vesicle to the uterus". Q. J. M. S., N. S. Vol. 24, 

 p. 657, Plate XXIV, Pig. 2. 



Anat. Anz. XVin. Aufsätze. 24 



