STUDIES IN MAMMALIAN EMBRYOLOGY. 333 
haut,’ p. 46) in the human placenta, where so-called “ zwie- 
belschalenartige” investments of certain decidual vessels are 
specially noticed by them in early and non-pregnant stages, 
but do not seem to be intermixed with other vessels without 
such sheaths as were noticed in Hrinaceus. Friedlander 
(‘ Physiol. anat. Unters. tiber den Uterus,’ 1870) has been the 
first to describe spontaneous thrombosis of uterine veins in the 
placentary region. Leopold (l. ¢., p. 88) brings further details 
concerning this process. I cannot gather from their descrip- 
tion points which would allow closer comparisons to be drawn 
between this and what was above noticed for the hedgehog. 
We now come to the decidual tissue itself. In the early 
stages (figs. 37—39) the cellular elements are bulky and poly- 
gonal, and the nuclei round and distinct. As pregnancy 
advances—the increase in size of the embryo inducing an in- 
creased stretching of the uterine walls—these cellular elements 
become more fusiform, and constitute the massive pseudo- 
muscular layer that was already described above (p. 324). It is 
against this tissue that—as we have noticed above—the deci- 
duofracts are applied ; it is in this tissue that the blood-vessels, 
both with and without a perivascular sheath, take their course 
towards the placenta. The name of fibrillar decidual tissue 
would seem to be best applicable to this region, which shows 
no considerable variation of histological aspect during the 
stages of pregnancy subsequent to that of diagram fig. 382, 
although its extension is diminished when the decidua reflexa 
has become membranaceous. It has the form of a cap over- 
topping the already convex upper surface of that region of the 
placenta that is formed of trophoblastic and allantoidean tissue. 
As such it forms a considerable part of the shed after-birth, as 
was already noticed on p. 331. This fibrillar decidual tissue, 
which is closest to the deciduofracts, often assumes a very 
puzzling aspect, all the more puzzling because it is not an in- 
evitable metamorphosis which the tissue always undergoes, con- 
sidering that sometimes it may be absent. This phenomenon 
generally includes a zone of a certain breadth, the inner 
surface being applied against the outer surface of the tropho- 
