PLACENTA IN PERAMELES 163 



structural modifications associated with the formation of the 

 allantoic placenta. 



A section through the very folded allantoic placental area 

 reveals the presence of an extremely homogeneous granular 

 layer with lobed lower margins, indicating the positions of the 

 original syncytial nests. 



Unfortunately, the gap between this and the preceding stage 

 is too wide to make it possible to closely follow and be certain 

 of the histolytic changes wdiich have taken place in the placental 

 region. 



Within the lobulated portions of the diploplasma are very 

 irregular groups of nuclei corresponding in position to the 

 groups of the original syncytiuin. 



An examination of such an aggregation at this stage shows 

 that it consists mainly of plasmodiblast nuclei which are in 

 a marked stage of degeneration. One simple group is shown in 

 fig. 26 of this paper, while many others are depicted by Hill 

 (1899, PI. xliii, figs. 6 and 7). 



One particular type of degeneration change in the case of 

 a plasmodial nucleus is shown in fig. 17. This corresponds 

 closely with what has been recorded by Jenkinson (1902, 

 figs. 24-5) for the megalokaryocytes of the mouse. The 

 original robust nucleus loses its contour through shrivelling 

 of the nuclear membrane and escape of the nuclear sap. The 

 chromatin becomes irregularly arranged. Thereupon the whole 

 nucleus flattens and becomes of the nature of an extremely thin 

 rod, which by further absorption is seen as a few darkly staining 

 particles in the general ground-mass, finally disappearing. 



There are to be found also degenerating vesicular elements 

 which no doubt are the remnants of the formerly numerous 

 maternal epithelial nuclei. On this important point, in view 

 of the paucity of the material available, I regret I am not able 

 to make any certain statement. 



If, however, this interpretation be correct, then Hubrecht's 

 suggestion (1909) that the placenta of Perameles consists 

 in its final stage of a ' mixed syncytium ' is not very far from 

 the truth. 



M 2 



