PLACENTA IN PERAMELKS 135 



are important. The younger one, a specimen of Perameles 

 o b e s u 1 a , shows the tirst fixation of the chorion ; the older one, 

 which l)elongs to the Tasmanian form. P. giinni. is a stage 

 in which the first attachment of the allantois is in progress. 



Both specimens were preserved in Hill's fluid (picro-nitro- 

 aceto-osmic). Sections were stained, sometimes with Ehrlich's 

 haematoxylin, sometimes by the iron-haematoxylin method, 

 counter-stained in each case by means of eosin. 



In addition, I have been able, by the courtesy of Messrs. 

 L. Harrison and B. A. Briggs of the Zoological Department 

 of the University of Sydney, to refer to several excellently- 

 preserved sections in the collection of that department and 

 representing some of the material mounted by Dr. Hill of the 

 stages described in his paper. This very important collection 

 is as follows : 



Perameles obesula, 7mm. stage, one microscope slide 

 containing one representative section of the uterine wall, 

 showing the placental and extraplacental areas and the attach- 

 ment of the allantois. 



Perameles obesula, 12-5 mm. stage, one slide with five 

 sections similar to the above. 



Perameles nasuta, post-partum stage, one slide with 

 one section. 



All the above are stained with haematoxylin and eosin. 



Hill's Stage D, representing embryos 8-8-75 mm., is, unfor- 

 tunately, not represented in the Sydney University collection. 



4. Terminology. 



The expressions ' omphalopleure *, ' vascular omphalopleure ', 

 and ' bilaminar omphalopleure ' first used by Hill are so con- 

 venient and expressive as to need no apology for their continued 

 employment. 



, The term ' chorion " or ' true chorion ' will be used by me 

 in the same sense as by Minot and Hill to indicate that part of 

 the extra-embryonic somatopleure which remains after separa- 

 tion of the amnion. 



I will follow the example of most embryologists in using 



