No. 3.] UTERUS AND EMBRYO. 357 
deserves special study. Over the area of attachment the uterine 
epithelium, Fig. 7, 4.ef, is degenerated as before described, its 
surface is extremely irregular, but the ectoderm, Lcéo, is per- 
fectly fitted to every irregularity, but the free surface (towards 
the mesoderm) is comparatively smooth; the layer consists of 
two, three, or four strata of cells. Beyond the area of attach- 
ment the ectoderm again thins out. 
As to how the tissues are held together, my observations 
afford no explanation. It seems to me possible that the two 
tissues actually grow together asa grafting unites with a bough; 
but for aught we know it may be by some other process, perhaps 
simple agglutination. The thickening of the ectoderm I am in- 
clined to regard as degenerative, and therefore somewhat com- 
parable to the degenerative thickening of the uterine epithelium. 
I am brought to this view by no conclusive observations, but 
chiefly by two facts: 1°, that in later stages the ectoderm seems 
to have disappeared over the greater part of the placenta (see § 
7. Uterus of eleven days); 2°, hyperplasia is often the com- 
mencement of degeneration, as is familiarly known to patholo- 
gists. To this evidence may be added the appearance of the 
ectoderm at nine days and seventeen hours, which I interpret 
as indicative of degeneration. 
§ 6. Uterus of nine days and seventeen hours. — In my 
specimen there are not many changes from the previous stage 
last described, but of these changes the following deserve special 
mention: 1°, the commencing formation of perivascular decid- 
ual cells in the peri-placenta ; 2°, the reconstitution of the ob-pla- 
cental epithelium; 3°, the formation of the true chorion; 4°, 
changes in the extra-embryonic ectoderm; 5°, the contents of 
the placental blood-vessels. 
1°. The peri-placenta is still only a small bolster at the side of 
the placenta ; its glands are still recognizable and its blood-ves- 
sels are more conspicuous; the connective tissue cells are en- 
larged and have begun to form more or less distinct coats around 
the blood-vessels. I feel assured that the decidual cells arise 
here in the same way as those of the outer zone of the placenta ; 
the cells in the two parts appear to me identical in character as 
soon as they attain their full development, and to differ only 
in the period during which their development takes place ; later 
on, Fig. 8, Pl. XXVIII., the peri-placenta forms, together with 
