EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 115 
(948, Figs. 31, 39, 40), is here represented by Figs. 146 and 
152. It has since then also been studied by Strahl (’99), and 
offers different points by which it is differentiated from that of 
the Ungulates, as, for example the presence of capsular spaces 
(Fig. 152) which have been discussed by Strahl in his con- 
tribution to Hertwig’s Handbuch. Chiromys (Fig. 151) has 
the same arrangement. We cannot for the present indicate 
the intermediate steps by which the simplification of a pla- 
centa of the Insectivorous or Primate type down to that of 
the present Lemurs was brought about and we may safely 
affirm that this secret has been taken into the grave by very 
old, probably mesozoic, Mammalia. But I hope that all the 
considerations we have discussed above may have sterilized 
any attempt to place Ungulates and Lemurs on one line, viz. 
that of the so-called primitive placentation. We are in no 
way justified to evolve the ever so much more intricate and 
perfectioned placental arrangements of Primates and Insecti- 
vores out of them. 
3. Didelphia Placentalia. 
We must now for a moment consider more closely the place 
which the placentiferous Didelphia have to occupy in this line 
of argumentation. 
Without it being necessary to recapitulate the details 
furnished by comparative anatomy we may take it for granted 
that these mammals, which are now restricted to Australia 
and America (but in the tertiary period also spread over 
Europe), ought to be looked upon as an early side-branch of 
the mammalian stem, which has undergone very numerous 
adaptations to food and surroundings in its recent home, and 
which is characterised by peculiar points, both osteological 
and odontological, but more particularly by the curious 
physiological process of short pregnancy and very early birth 
that is followed by a protracted period of passive adhesive- 
ness to the maternal nipple, generally inside a ventral brood- 
pouch. 
Besides scanty details about their development which we 
