4 
INTRODUCTION 
or less degree, of “ inversion of the germinal layers.” Two methods of 
imbedding, which will be dealt with in greater detail later, have been 
described in animals with a decidua capsularis. In the hedgehog, mice, 
and rats, the ovum is said to be received into a recess or fissure of the 
mucous lining of the uterus; the epithelium disappears round the blasto¬ 
cyst ; the mucous membrane becomes greatly thickened to form the 
decidua capsularis; and the fissure is cut off from the general cavity of 
the uterus by the fusion of the lips of the decidual swellings from which 
the epithelium has likewise vanished. In the guinea-pig the observations 
of Graf v. Spee seem to prove that the ovum, while still in the early 
blastocyst stage, destroys the epithelium of the surface at the spot 
where it becomes implanted, by the activity of its ectodermic cells, 
and then, by a continuance of the process of destruction and solution, 
imbeds itself in the connective tissue of the mucosa. One or other of 
these alternatives must apply to the human ovum, and we submit our 
two communications as a contribution towards the solution of the problem. 
In regard to the initial stages of placentation a very large body of 
data has been accumulated by comparative embryology, and our views as 
to the deciduate placenta have undergone considerable modification. Apart 
from the general character of the placentation in the different orders of 
mammals, debate has centred on the nature of certain layers of cells 
which separate the foetal from the maternal blood in the placenta. It is 
unnecessary here to enter on any detailed account of the various and 
contradictory opinions which have been held on this histological detail, 
or of the several theories which have been put forward on the subject of 
the origin of these layers. 1 
It is now very generally admitted that the evidence afforded by both 
human and lower mammalian material is in favour of the foetal, i.e. 
chorionic origin of both layers covering the villi in the human placenta. 
No doubt appears to exist in the mind of anyone as to the cellular layer, 
generally known as Langhans’ layer, but there is still a lack of decisive 
1 The different hypotheses are fully set forth by Hubert Peters, by Webster ( Human Placen¬ 
tation), and by Strahl ( Her twig 1 s Handbuch der EntwicJcelungslehre ) ; they have also been dealt 
with by Teacher in his papers on “Chorion-epithelioma,” and are briefly summarised by Bryce in 
Quain’s Anatomy , vol. i. 11th ed. 1908. 
