Phenomena tn some Tasmani<iti Mursupialfi. 229 



one birth (extrenioly raioly it is one). More often thore ure 

 more, the niaxiinuiu number observed being six, throe in each 

 uterus. On two occasions I have touiul three young in the 

 poucli, just after parturition, two of which were attached, the 

 remaining one dead and shrivelled. In tin's resj)ect, then, 

 Pseui/ochints exhibits a condition approximatinn; to the 

 inimitive one so far observed oidy in polyprotodont forms. 



With the growth of the pouch-young the niarsupium 

 increases in size and develops two lateral extensions — one on 

 each side — which pass forward in the manner indicated by 

 the broken line in the text-figure. Each of these extensions 

 shelters an embryo, normally head downward. 



In the process of parturition in Psexuiochirus the distal 

 portion of the yolk-sac, as in other marsupials (Stirling^, 

 Ilartman t> Hill I), remains behind in the uterus, while the 

 proximal portion is drawn out into a long tenuous tube which 

 can be followed by careful dissection into and along the 

 pseudo-vaginal passage. Practically the whole of the yolk- 

 sac, then, remains behind to be absorbed — a condition to 

 which the term " contradeeiduate" can be applied with as 

 much justification as in the case of Perameles, Dast/uniSjiind 

 Talpa. 



Coalescence of the bilaminar omphalopleure of neighbouring 

 embryos in the uterus of Pseiidochirus is not uncommon, as 

 is also recorded in the case of Dasyurus by Hill and for 

 Diiitlphi/H by Osborne §. 



Examples of true twins in marsupials appear to be rare. 

 Patterson and Ilartman || have recently drawn attention to a 

 case of polyembryony in the American opossum, in which 

 four early embryos were discovered in the one blastocyst. 

 Of these, however, only one appeared to be normal. 



Bluntschli ^ has also reported a supposed case of poly- 

 embryony in Didelpliys marsupial'is, a condition which he 

 appears to regard as normal for this species. In tliis case all 

 the four embryos were normal and fairly well advanced. It 



* Stirling, E. C. " On some Points in the Anatomy of the Female 

 Organs of Generation of the Kangaroo, etc.," P. Z. S. 1889. 



t Ilartman, loc. oil. (1920). 



J Hill, J. P., "The Placentation of Perameles" Quart. Juurn. Micr. 

 Sc. vol. xliii. (1899). 



§ Osborne, 11. F., "The Fffital Membranes of the Marsupials: The 

 Yolk-pac Piacentft in Dide/p/ii/s'' Journ. Mnrphology, vol. i. (18^8). 



II Patterson and Ilartman, "A Polyembryonic Blastocyst in the 

 Opossum," Anat. Kec. vol. xiii. (1917). 



51 niuntschli, H., " Zur Entwickelung?>ge9chichte .... von Didelphys 

 marsupiulis, etc.," \'erhandl. d. Anatom. Oesellsili. ((ireifswald), Jeiia 

 (1913). 



