120 A. A. W. HUBRECHT. 
a placenta in case of certain Didelphia (Perameles) and deny 
it to certain Monodelphia (Equus, Sus, Nycticebus, Galago, 
and others). Attempts at systematic arrangements based 
on placental characters having never been very successful 
up to now, there is no objection to this somewhat radical 
change in our conceptions. 
And so in order to understand the final constitution of the 
placenta it is not sufficient to be acquainted with the very 
varied changes in the trophoblast which precede it, but it is 
also necessary to study most closely the diverse modifications 
and proliferations which take place in the maternal mucosa 
preparatory to the coalescence with distinct regions of the 
embryonic trophoblast. It would fall outside the scope of 
this paper to enter into a full and detailed description of 
all these varied modifications. I will only select a few 
examples in order to call attention to the extreme width of 
variation which this series of maternal preparatory arrange- 
ments for the confluence with the semi-parasitic trophoblastic 
tissues can undergo in different genera of mammals. 
But before entering upon those details I wish to have 
formulated a generalisation to which a close comparison of all 
the facts observed in this whole field of inquiry necessarily 
leads us. ‘Those facts then have established that the quintes- 
sence of the respective changes that become apparent in the 
maternal tissue consists in: (a) degeneration and destruc- 
tion—sooner or Jater—of the uterine epithelium and of the 
uterine glands in the region of the future placenta; (b) 
increase of the vascular supply in that region; (c) production 
of tissues histologically resembling as closely as possible 
those which the trophoblast produces; fusion and concre- 
scence being thus facilitated; (d) arrangements by which 
extravasation of blood in other directions than that of the 
trophoblastic lacune is rendered difficult or impossible ; 
(e) in certain cases development of haematopoietic properties, 
the blood-corpuscles thus formed being set free in the 
maternal blood as are those produced by hematopoietic 
processes in certain trophoblast cells; (/) arrangements by 
