STUDIES IN MAMMALIAN EMBRYOLOGY. 315 
the combined action of two processes, viz. (a) the soldering 
together of the two opposite walls of the depression; (4) the 
formation of a blood-clot, insignificant in the beginning, but 
gradually becoming more massive, which is sweated out into 
the uterus lumen, in consequence of a peculiar hemorrhagic 
cedema. ‘This cedema is most distinctly noticed in the earliest 
stages along the lips of the groove (ef. figs. 2 and 87). In this 
region the proliferating blood-vessels very soon become con- 
spicuous as spacious lumina, that carry a strong flow of blood 
towards these lips. The latter, when the uterus is cut open in 
the fresh animal, are, indeed, easily recognised by their deep 
red, congested aspect. The process of formation of the blood- 
clot is not wholly limited to the lips, but it also takes place 
lower down, along the sides of the narrow groove. In sections 
blood-extravasates can often be noticed as passing between the 
epithelium cells, sometimes heaving these out of their place 
(Pl. XX, fig. 37). In further stages this blood-clot is seen to 
contain numerous remnants of cells that have thus become 
detached and enclosed during its formation (fig. 39). After 
a short time the sides of the groove approach each other, more 
especially in the vicinity of the lips, the clot in consequence 
assuming a more conic shape in transverse section with the 
broad end turned downwards, in the direction of the blastocyst 
(figs. 3, 7, and 89). Definite fusion of the two opposite lips 
of the groove is at last reached, the superficial tissue becoming 
at the same time more congested with blood than ever, and 
remnants of the primary blood-clot being distinctly traceable, 
both between the fused lips of the groove and the blastocyst, 
and outside of the former, 1. e. in the uterus lumen. Further- 
more, there is an accumulation of maternal blood between the 
elongated clot and the walls of the original depression (e, e, in 
fig. 39) right and left of it. It appears to me that, after occlu- 
sion of the lips maternal blood is not only accumulated, but 
may also circulate in these spaces, until in later phases, hereafter 
to be described, all circulation ceases in these regions. As 
the blastocyst increases in size the portion of the proliferated 
decidual region which we have here considered—i, e, the coalesc- 
