No. 410.] | THE MAMMALIAN SUBCLASSES. 127 
this animal and the modifications of the allantois in other 
marsupials. 
Prior to Hill’s discovery of the placental allantois of Pera- 
meles, other investigators! had examined the modifications of 
the allantois in certain other forms. Owen, Selenka, and 
Semon had described for the phalangers (Phalangeridz) and 
kangaroos (Macropodidz) (Fig. 2, c) one condition in which 
the allantois was reduced and enclosed by the yolk-sac. Selenka 
had found a similar condition in the opossum (Didelphyidz). 
Caldwell and Semon, on the other hand, had found in the 
koala (Phascolarctos) (Fig. 2, 4) another condition in which 
Fic. 2, a~c.—The main modifications of the marsupial allantois arranged in the order of their 
primitiveness as interpreted by Hill. (č may represent the type from which æ and c have 
been derived.) ; : 
4, placental allantois of Perameles (from Parker and Haswell, after Hill); 4, “respiratory 
allantois of Phascolarctos cinereus; c, the reduced allantois of Æpyprymnus rufescens 
(Macropodidz), (4 and c after Semon); y.s., yolk-sac; aZ., allantois. 
the allantois was well developed and freely exposed to contact 
with the embryonic envelope, much as in the Reptilia. 
In his concluding remarks Hill considers these types and 
endeavors to decide as to which one of them is the most primi- 
tive; in other words, to determine whether a placental allantois 
is being developed secondarily in the Marsupialia, or whether 
the placental condition was formerly present throughout the 
group and has been subsequently lost in all forms (so far as 
known) but one. | 
He argues thus: that the reduced condition of the allantois 
is necessarily secondary; that although it is possible to derive 
1 See Hill ('97, p. 438)- 
