PLACENTA IN PERAMELES 141 



Over the area of fixation to the uterine syncytium, however, 

 an important and highly signiticant alteration has been im- 

 pressed upon the ectoderm I)y which it becomes converted into 

 a typical trophoderm (Minot) or ectopia cent a (Duval) 

 so characteristic of this layer in the Eutheria. 



At certain points in this portion of the trophoblast, cell 

 proliferation takes place. The cells of the original layer divide 

 to give rise to nucleated groups in which the cell outlines have 

 disappeared. These cytoplasmic aggregations possess an 

 irregular contour due to the presence of pseudopodial pro- 

 cesses, so there is distinctly present here a layer definitely 

 homologous with the plasmodial structure (plasmodium, 

 p 1 a s m o d i b 1 a s t , or p 1 a s m o d i t r o p h o b 1 a s t) so charac- 

 teristic of the placentation of the Eutheria. The appear- 

 ance of the plasmodiblast at this stage is shown in figs. 4, 5, 

 and 6. At various points the plasmodial nuclei invade the 

 uterine syncytium. The soldering of the foetal trophoderm to 

 the maternal syncytium is brought about by the above- 

 mentioned pseudopodial processes, in the meshes of which 

 numerous spaces are enclosed. The remaining basal cells of the 

 trophoblast layer form the cyto blast or cytotropho- 

 blast. This is by no means at first a definite cell-layer. It 

 is apparenth^ not till a little later that the basally-situated 

 nuclei divide in a regular way to form the more definite cellular 

 layer known as the cytoblast. 



A point of the greatest significance, and one to which I shall 

 later refer, is the fact that the localities of proliferation are 

 determined by the presence of the syncj^tial nests, and it is 

 into these that the plasmodial masses pass. Fig. 4 shows this 

 phenomenon, while it is also indicated in fig. 8, in which, 

 however, the nests are not quite cut centrally. The effect of 

 the growth of the attached trophoblastic cells on the maternal 

 structures is shown in figs. 4 and 7. In fig, 4 the chorionic 

 ectoderm cells show l)ut little departure from their original 

 linear arrangement, but have already begun to give off pseudo- 

 podial processes which immediately phagocj^tically attack the 

 syncytial nuclei of a neighbouring syncytial lobule, only part of 



