128 T. THOMSON FLYNN 



It need hardly be said that in these respects the placentation 

 of this animal, as interpreted by Hill, is extremely disappointing. 



The combination of a persistent uterine syncytium with 

 a degenerating chorionic ectoderm is without parallel in the 

 Eutheria , and any attempt at a comparison of the placenta- 

 tion of the two groups results in a deadlock. 



Although Hill's results have been accepted by many, never- 

 theless Hubrecht (1909) ventures to question whether, morpho- 

 logically, the placenta of Perameles will not prove on further 

 investigation to he more comparable with some one or other 

 of the various placental styles found in the Eutheria. And 

 that there is some foundation for Hubrecht 's opinion is un- 

 doubtedly apparent from an examination of Hill's figvires 

 alone. 



Before passing to a consideration of this question it will 

 perhaps be necessary to have before us a short account of some 

 special details of the placentation of this marsupial as described 

 by Professor Hill. 



His most important and peculiar point concerns the history 

 of the foetal ectoderm. The portion of this which is concerned 

 in the fixation of the embryo is said t(j disappear almost 

 completely, being represented in the l'2-5 mm. stage by a few 

 scattered cells in the original ectodermal position. The dis- 

 appearance is stated to ))e of the nature of a degeneration, this, 

 apparently, not being inaugurated until the allantoic attach- 

 ment has taken place and the allanto-chorionic fusion com- 

 pleted. In the earliest placental stage (Stage C, 7 mm.) the 

 allantoic placenta is already on the way towards full establish- 

 ment, the allantoic vesicle being already attached to the chorion 

 by its placental face. Excellent figures are given of this stage 

 (1897, Pis. xxix and xxx, figs. 5 to 12). 



At a later stage (D, embrj^os 8-8-75 mm.) the ectoderm has, 

 apparently, almost completely disappeared. In the description 

 of Stage C, it is stated that the cells towards the central portion 

 of the foetal ectoderm are ' of a very varying size and shape, 

 and, in places, through the disappearance of the outlines 

 between adjacent cells, large multinucleate cells have been 



