DEVELOPMENT OF THE OPOSSUM 3 



account of the cleavage of the egg of Dasyurus, our knowledge 

 of the early stages of marsupials rested largely on Selenka's ('87) 

 work on the opossum. Because of the prominence of this author's 

 famous 'Studien, in which the account was published, Caldwell's 

 ('87) interpretation of the outer envelope of the marsupial egg 

 as the shell membrane was completely overlooked until recalled 

 by Hill. Inasmuch as his illuminating results cast doubt upon 

 the validity of Selenka's interpretations, as well as the accuracy 

 of his descriptions, which seemed to be based upon insufficient 

 material, it seemed desirable to study the American marsupial 

 in this locality where sufficient fresh and normal material could 

 be secured. The problem was suggested to the writer by Dr. 

 J. T. Patterson and the work was done under his direction. I 

 am pleased gratefully to acknowledge his helpful suggestions 

 throughout the progress of the work. 



b. Review of the literature on the early history of the marsupial egg 



1) The work of Caldwell and Selenka. In 1883-4 Caldwell 

 collected a considerable amount of early marsupial and mono- 

 treme material in Australia. In 1884 he published several pre- 

 liminary communications in which he recorded the discovery 

 of the yolk mass of the monotreme egg. This discovery greatly 

 increased Selenka's interest in marsupial embryology, in the 

 study of which he had been for sometime engaged when Cald- 

 well's paper appeared. Caldwell must also be credited with the 

 discovery of the 'shell' of the marsupial egg; for he correctly 

 homologized the 'Hhin transparent membrane, 0.0015 mm. thick," 

 of the marsupial, Phascolarctus, with the shell membrane of the 

 monotreme egg. This interpretation Selenka completely ignored, 

 for he mistook the shell membrane of the opossum for the corona 

 radiata. 



After several years of fruitless experiments in attempting to 

 rear Australian and Brazilian species, Selenka imported a hun- 

 dred or more individuals of the 'hardy' North American species, 

 Didelphys virginiana, two females of which furnished the 16 

 eggs that formed the basis of his account (Selenka, '87) of cleav- 

 age and blastocyst formation. 



