THE PLACENTATION OP FERAMELES. 393 



Another highly significant fact in connection with the 

 syncytinra is that it is already vascularised. Both hetvveen 

 the lobules and enclosed in the protoplasm of the syncytium 

 itself small capillary vessels^ with distinct nucleated endothelial 

 walls and containing maternal blood-corpuscles, can be readily 

 made out (fig. 4, syn. c). These syncytial capillaries are 

 derived from the capillaries of the mucosa, which are seen to 

 pass up between the syncytial lobules, and from there to 

 ramify out in the syncytium itself. That the capillaries 

 actually penetrate into the syncytium by their own growth 

 seems beyond question, but no doubt the subsequent gradual 

 enlargement of the syncytium as a whole, and especially of its 

 lobules, also aids in bringing about the enclosure of the ca- 

 pillaries. 



We can only regard the formation of the syncytial lobules 

 as the result of the enlargement and growth of the protoplasm, 

 and it seems probable that the direct invasion of the syncytium 

 by ingrowing capillaries may have been the inciting cause of 

 this mode of growth. 



This transformation of the uterine epithelium into a vascular 

 syncytium is a highly distinctive and peculiar feature in the 

 developmental history of the placentation of Perameles. Such 

 a condition has hitherto never been met with in any other 

 mammalian form, and is especially interesting in view of the 

 wide-spread occurrence of degeneration of the uterine epithe- 

 lium prior to placental formation in so many diverse Eutherian 

 orders. The only form known to me which in the behaviour 

 of its uterine epithelium offers any points of analogy with the 

 above-described transformation of the epithelium in Perameles 

 is Sorex. Hubrecht has shown that, in this Insectivore, mo- 

 dification of the uterine epithelium over the placental area, by 

 way of proliferation, is the first and most important change 

 " that takes place in the maternal tissues preparatory to the 

 reception, fixation, and nutrition of the blastocyst^' (7, p. 491). 

 But when one compares the details of the proliferation in the 

 two cases they are seen to be essentially different in character, 

 though offering interesting analogies. In Sorex, following 



