THE EARLY DEVELOPMENT OE THE MAKSUPIALIA. 5 



To Miss V. Sheffield I atn indebted for the origfinal o£ fio-. 63. 

 To my friend Dr, F. P. Sandes, Sydne}^ I am indebted for 

 kind help in the revision of certain parts of the manuscript. 



Chapter I. — Critical Review of Peeviods Observations ox 

 THE Early Development of the Marsupialia. 



Apart from the very brief abstract of a sliort paper on the 

 development of Dasjurus, which I read before Section D of 

 the British Association in 1908 (included in Dr, Ashworth's 

 Report, 'Nature/ vol. Ixxviii), our knowledge of the processes 

 of cleavage and germ-layer formation in the Marsupialia is 

 based (1) on the well-known observations of the late Emil 

 Selenka ('86) on the development of the A^irginian opossum 

 (Didelphys marsupialis), published in 1886 as Heft 4 of 

 his classical ' Studien'; and (2) on those of W. H. Caldwell 

 ('87) on the uterine ovum, and cleavage process in the native 

 bear (Phascolarct us cinereus). 



Selenka's account of the mode of origin of the germ-layers 

 iu Didelphys differs Avidely, us the sequel will show, from my 

 description of the same iu Dasyurus. Now Didelphys and 

 Dasyurus are two marsupials^ admittedly allied by the closest 

 structural ties, and we should therefore not expect a priori 

 that they would differ fundamentally in the details of their 

 early ontogeny, however much they might diverge in respect 

 of the details of their embryonal nutritional arrangements. 



Furthermore, we might reasonably hope, in view of the 

 generally admitted relationships of the Marsupialia, that a 

 knowledge of their early development would aid us in the 

 interpretation of that of Eutheria, or, at least, that their 

 early developmental phenomena would be readily comparable 

 Avith those of Eutheria. It cannot be said that Selenka's 

 observations realise either of these expectations, " Which- 

 ever view is taken of Selenka's description of the opossum," 

 writes Assheton ('98, p. 254), ''many obvious difficulties 

 remain for the solution of which no satisfactory suggestion 

 can as yet be offered." 



