140 A. A. W. HUBRECHT. 
events and the different phases in the mutual relations of 
blastocyst and mucosa ought to be fully known in order to 
furnish us with all the data that can be brought to bear upon 
this important question. And it is to be fervently hoped 
that those genera that are very rapidly diminishing in 
number in their native land, some of them even on the verge 
of disappearance, may yet be fully investigated before they 
have been exterminated, and have thereby become as mute 
on this important point as are their fossil predecessors. 
Turning back to the Monodelphia we notice that among 
the Insectivores another genus than the mole, above dis- 
cussed, furnishes particular points of comparison with certain 
Didelphia. ‘The genus I here allude to is Sorex, in which a 
localised strong proliferation of the uterine epithelium has 
been described by me (944, Figs. 74 and 80) into which 
allantoidean villi fit, which in that early stage very much 
resemble those that have been figured by Hill (98, Pl. 33, 
fies. 28, 29) for Perameles. If the pregnancy of Sorex were 
to be brought to an early close in this very stage by a series 
of new adaptations as have occurred in the Didelphia the 
resemblance on general points between Sorex and Perameles 
would be certainly remarkable. ‘he maternal epithelial 
proliferation of Sorex, however, does not give rise to a 
syncytium as in Perameles, but to a cellular agglomeration 
in which crypts appear, each of which harbours a tropho- 
blastic villus with its core of vascularised allantoidean 
tissue. 
The parallel cases which we have been able to institute 
between Didelphia on the one hand, certain Insectivores and 
Carnivores on the other hand, seem to encourage us when we 
pretend that a similar stage must have been the average 
degree of complication to which the earliest mammalhan 
placentation corresponded, and that the so-called diffuse 
placentation which up to now has been lcoked upon as 
representing an early starting-point has wrongly usurped 
this place, as we will by-and-by demonstrate when we will 
advocate that the latter arrangement is an example of a 
