130 
here interpolated in the ontogeny before the specialization of the 
allantois, which functions as the vascular pathway between the chorion 
and embryo, both primitively and permanently. The enlargement of 
the allantois in ungulate mammals is a supervening change, effected 
perhaps by an atavistic recurrence to reptilian ontogeny. 
RypeEr (8) has advanced the theory that the zonary placenta is 
older than the discoidal, but Minor (7), 434, has shown that this 
view is untenable. 
The degeneration changes in the uterus occur so far at present 
known only in connection with the chorionic placenta; in the ungulates 
the uterine mucosa is modified in structure in connection with the 
development of the placenta, but the modifications are not known to 
be degenerative; hence in the allantoic placenta the maternal blood 
flows in maternal blood vessels and it is always separated by maternal 
connection tissue and epithelium from the chorion. 
Theory of the placenta. According to the views explained 
in the preceeding pages, I hold the placenta to be an organ of the 
chorion; that primitively the chorion had its own circulation, and 
formed the discoidal placenta by developing villi which grew down 
into the degenerating uterine mucosa; by the degeneration of the 
maternal tissues the maternal blood is brought closer to the villi, and 
the degeneration may go so far that all the tissue of the uterus 
between the villi disappears; a layer of the mucosa is preserved be- 
tween the ends of the villi and the muscularis uteri to form the so 
called decidua; the placenta receives its foetal blood by the means 
of large vessels running in the mesoderm of the allantois. From this 
discoidal chorionic placenta the zonary placenta of carnivora, the 
diffuse placenta of the lower primates, and the metadiscoidal placenta 
of man have been evolved. 
A second type of placeuta, perhaps evolved from the first is found 
in ungulates, and is characterized by a vascular allantoic vesicle uniting 
with a now vascular chorion to form the foetal placenta, and by the 
absence of degeneration in the maternal tissue. This type is the 
allantoic placenta, which offers many interesting modifications. 
Literature cited. 
1) van Benepen, E., et JuLın, Cu., Recherches sur la formation des 
annexes foetales chez les mammiferes (Lapin, Chiroptéres). Archiv. 
de biol., V, 369—434, Pl. XX—XXIV, 1884. 
2) Bonner, R., Beiträge zur Embryologie der Wiederkäuer, gewonnen 
am Schafei. Arch. f. Anat. u. Physiol., Anat. Abtl., 1889, 1—106, 
Taf. I—VI. 
