INTRODUCTION 



The Virginia opossum (Didelphys virginiana Kerr) l is 

 very abundant and easily caught in almost every part of the 

 United States from about the latitude of Pennsylvania south. 

 Being a marsupial, and an extremely primitive one, it would 

 have been studied very thoroughly and used very widely for 

 laboratory purposes long ago, if it had been more amenable 

 to domestication. 



The first serious attempt to breed this animal in captivity 

 was that of Dr. Emil Selenka, who imported a large number of 

 them to Erlangen, Germany, some time just prior to 1887. 

 He had the good fortune to obtain several litters of eggs and 

 embryos before the colony went sterile and the animals died. 

 Pie tried the same experiment also with the South American 

 opossum, Didelphys cancrivorous, and with three Australian 

 marsupials (Hypsiprymnus penicillatus, Phalangista vulpina, 

 and Phalangista orientalis) ; but in these ventures he met with 

 less success than he had had with the Virginia opossum. 



Since his time several American zoologists and embryolo- 

 gists have tried to establish breeding colonies of Didelphys, 

 and all have met with failure until The Wistar Institute in 

 1930 decided to invest a considerable sum of money in a large- 

 scale effort to solve the problem. After several years of dis- 

 appointment the major difficulties seem at last to have been 

 overcome, and for the convenience of those who may want to 

 breed the opossum in their own laboratories I shall pass on 

 what information we have gained. 



The first essential is to prevent the animals from developing 

 rickets. Though from the very inception of our colony all the 



1 Another spelling of the generic name often seen substitutes an ' i ' for the 

 'y' in the last syllable. Etymologically this change makes the term mean 'two 

 dolphins or porpoises.' I prefer the spelling which means 'two uteri.' 



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