160 T. THOMSON FLYNN 



Here it can be seen that the arrangement of the syncytial 

 nuclei of the extra-placental area is similar to that of the 

 preceding stages. The presence of an internal space, free of 

 nuclei, in the syncytial nest should be noted. 



Passing from this region into the placental area the gradually 

 increasing activity of the proliferating chorionic ectoderm 

 becomes evident. More and more of the foetal nuclei occupy 

 the nests. The plasmodial nuclei in a large number of cases 

 form multinucleate groups. In many cases there are but two 

 nuclei in each group, these being comparable to the binucleate 

 cells ' diplokaryocytes ' described for some Eutheria (for 

 example in Ungulates, Assheton, 1 906) . More oft en 

 three or more nuclei are contained in one plasmodial mass. 



An examination of a more central section shows that in this 

 portion of the placental area many nests are now quite filled 

 with the plasmodiblast nuclei (fig. 18). So closely are these 

 packed in the nests that instead of being oval or irregularly 

 shaped they take a polygonal form due to mutual pressure. 

 The condition of the original epithelial nuclear nests can be 

 gauged from the fact that of twenty-seven observed in one 

 central portion of the allantoic area four were untouched ; 

 eleven were partly, twelve completely filled by invading 

 nuclei, the nests having the appearance of solid multinucleated 

 masses. 



In such positions also it is that the original cytoblast \a,jer 

 has practically disappeared. It is evident, therefore, that 

 a large proportion of the maternal epithelial tissue has been 

 replaced by intruding foetal material. One effect of this is that 

 the maternal capillaries of the allantoic area become enclosed 

 by the advancing plasmodium, and in many cases are now to be 

 found at the surface of the diploplasma, where they directly 

 underlie the allantois and even come into contact with the 

 allantoic vessels. 



Pigment is not so common as it is in the previous stage but 

 it is still to be found, particularly in leucocytes and in the 

 cytoblast cells. Here and there in the plasmodium are to be 

 seen isolated patches of pigment pointing to an active ingestion 



