PLACENTA IN PERAMELES ' 165 



the uterine wall possess the usual columnar epithelium and 

 are also ciliated. They are narrower than the glands opening 

 into the placental area. 



Seeing that the newly-born young of Perameles measures 

 but 14 mm. in length, the condition of the allantoic placenta 

 in the 12-5 mm. stage may be accepted as being practically 

 that of the full-term placenta. 



Stage 6 . Perameles n a s u t a , p o s t - p a r t u m . 



With regard to this stage the only way in which I can 

 supplement Hill's description is by pointing out that the 

 amount of foetal tissue left behind in the uterus is considerably 

 more than would be the case if Hill's conception of the placenta- 

 tion of this animal were correct. Instead of consisting merely 

 of the allantois — with the addition of a few remaining foetal 

 cells left behind after the degeneration of the chorionic ecto- 

 derm — there is really comprised in the contra-deciduate 

 portion, in addition to the allantois, the whole thickness of the 

 diploplasma, of which undoubtedly the greater part is foetal. 



6. Summary of Conclusions. 



The conclusions arrived at in the preceding pages may be 

 summarized as follows : 



1 . A 1 1 a n t o i c p 1 a c e n t a. 



(a) The fixation of the embryo is brought about by means 

 of the chorionic ectoderm at a time when the embryo measures 

 about 6 mm. direct length. 



(b) The chorionic ectoderm develops directly into two 

 portions, (1) a basal cellular cytoblast which by proliferation 

 gives rise to (2) a plasmodiblast. 



(c) The plasmodiblast phagocytically attacks the maternal 

 tissues, particularly the maternal nuclear aggregations or nests. 



{d) The proliferating foetal ectoderm and maternal syncytium 

 are thus intimately fused to form a structure, the diplo- 

 plasma. 



(e) Sooner or later most of the cytoblast layer disappears, 

 being converted into plasmodiblast. 



