PLACENTATION OF THE SEALS. 293 
third, and an equally large smooth surface was found at each pole. The zone 
on the chorion was now so completely interlocked with the corresponding zone 
in the uterine mucosa, that the two surfaces could not be detached from each 
other. The placenta could only be separated by rupturing the slender marginal 
_band of decidua reflexa, and tearing through or altogether pulling off the placental 
area of the mucosa, which area was intermediate between the placenta proper 
and the muscular coat of the uterus, and formed a well-defined decidua serotina. 
The villi of the chorion had the form of broad sinuous leaflets, which became 
attenuated at their uterine ends and gave off bud-like offsets from the free 
border. When vertical sections were made through the placenta, the villi were 
seen to pass vertically through the organ up to its uterine aspect. The trabeculee 
of maternal tissue, which formed the walls of the pits or crypts in which the villi 
were lodged, passed between the villi up to the chorion, and closely followed 
the sinuosities of the villi, so as to form an intimate investment for them; and- 
in horizontal sections through the organ they were seen to be arranged as a 
series of laminz, winding in a very sinuous manner between the leaf-like villi 
(fig. 14). Between the placenta proper and the muscular coat was a well- 
defined layer of serotina, equal in thickness to the muscular coat itself. It was 
traversed by the numerous blood-vessels which passed into and out of the 
placenta, and which formed not unfrequent anastomoses with each other. The 
decidua serotina consisted not only of the vascular connective tissue, but of the 
epithelial cells of this part of the mucosa, which were similar in character to 
those described in the preceding stage of development. In thin sections, tubes, 
lined by an epithelium, were seen cut transversely or obliquely; they were about 
equal in diameter tothe gland tubes seen in the serotina in a less advanced 
stage of gestation, and were without doubt the dilated glands of this portion of 
the mucosa. It may here be stated, that in the non-placental area of the same 
uterus the tubular glands were distinctly seen separated from each other by 
comparatively wide intervals of interglandular tissue. The chorionic villi dipped 
into depressions in the decidua serotina, and were in contact with its epithelium. 
The trabecule and laminz situated in the substance of the placenta were also 
continuous with the serotina, and were invested by an epithelial layer, the cells 
of which were modified columns, like the cells of the decidua serotina. The 
blood-vessels of the serotina entered the laminz and trabecule, and ramified in 
them throughout the maternal part of the placenta. In the placenta of one of 
the embryos where the maternal vessels were injected, they formed a network 
of capillaries of ordinary magnitude. In the other placente from the same 
uterus, the maternal capillaries, when injected with red gelatine, were dilated to 
two or three times the size of the capillaries in the foetal villi, and ascended 
almost vertically in the trabecule (fig. 13). Not unfrequently near the chorionic 
surface they dilated into sinus-like enlargements, which were crowded with 
