of Edinburgh, Session 1874-75. 
543 
shedding or non-shedding of maternal tissue, along with the foetal, 
during the act of parturition, is determined by the degree of inter- 
locking of the foetal and maternal portions of the organ during 
the formation of the placenta, and not from the presence in the 
deciduata of a structure or structures which do not exist in the 
non-deciduata. In both forms the same anatomical elements 
exist, though, as in the case of the human placenta, the maternal 
constituents may become so modified in arrangement as greatly to 
obscure their original characters. The foetal part of the placenta 
consists of a chorion more or less perfectly covered with vascular 
villi: the maternal part consists of a modified uterine mucosa, the 
surface of which is composed of the modified epithelial cells of the 
mucous membrane, beneath which is a highly vascular connective 
tissue, the modified sub-epithelial connective tissue of the mucosa. 
In those animals in which the chorion remains almost entirely 
covered by villi, as in the pig, mare, and cetacean, the villi are short, 
with simple branches, and the depressions, pits, or crypts in the 
uterine mucosa for their reception are consequently shallow. Dur- 
ing the act of parturition the villi are so readily liberated from the 
uterine crypts that no maternal tissue is necessarily shed along with 
them, though even here it is not difficult to see that, should the 
epithelial serotina from any cause become detached from the sub- 
epithelial connective tissue, flakes of it might pass off along with 
the villous chorion. In the zono- and disco-placentary mammals, 
where the villi are much longer, and, as a rule, much more exten- 
sively branched, the constituents of the mucosa, both epithelial 
and sub-epithelial, are so intermingled with the foetal villi in the 
region of the placenta as, as is generally admitted, to be shed along 
with them. The ruminants, therefore, with their scattered coty- 
ledons, are seen to occupy, as regards deciduation, an intermediate 
position between the animals with a diffused placenta on the one 
hand, and the zono- and disco-placentary mammals on the other; 
for whilst the former are apparently non-deciduate during the act 
of parturition, and the latter part with both the epithelial and the 
vascular sub-epithelial constituents of the uterine mucosa in the 
placental area, the ruminants shed, as a rule at least, only the 
epithelial lining of the uterine pits into which the foetal villi are 
inserted. 
