No. 3.] UTERUS AND EMBRYO. 369 
this statement is correct beyond any doubt, for I have several 
sections, in each of which the direct passage is observable under 
the microscope without even displacing the slide. By hypothe- 
sis this mesoderm is, however, really separated by a very thin 
covering of foetal ectoderm from the uterine tissue, and the 
whole constitutes a system of villi which have grown down like 
roots into the placental soil. 
That there is no communication between the foetal and ma- 
ternal circulations must be deduced from the fact that the two 
bloods are never mingled in one vessel, although found side by 
side in adjacent vessels. The separation of the foetal and ma- 
ternal blood has already been insisted upon. The full elucida- 
tion of the double placental circulation must be left for injec- 
tions to bring. 
In brief: The rabbit embryo is attached to the placenta by 
the ectoderm, which disappears from the surface of the placenta 
during the eleventh day ; the vascular connective tissue of the 
allantois grows probably by forming true villi into the placenta, 
and so comes close to the maternal circulation. 
In other rodents the placenta contains foetal vessels; its sur- 
face is covered after a certain stage by a thin epithelium like the 
mesothelial layer of the rabbit, and by a layer of vascular con- 
nective tissue. Hence it seems probable that the structure in 
the rabbit is typical of the class — compare § 12. 
§ 9. Uterus at thirteen days and three hours. — The pla- 
centa and embryo are considerably bigger than at eleven days, 
but the structure of the parts is comparatively little changed. 
A complete section is drawn in Fig.9. The longitudinal 
muscles, Z.7z., and the circular muscles c.m., form the external 
covering. They differ in microscopical appearance from the mus- 
cles of the resting uterus, but I have not investigated the 
change in them. 
The placenta is very bulky. Its two lobes have begun to 
form separate protuberances, so that the top of the placenta is 
no longer a nearly plane surface. The placental surface is cov- 
ered by the mesothelium, which is a little thicker than in the 
previous stage, the cells having a greater vertical diameter. 
Between the mesothelium and the glandular layer, g/, is the 
vascular mesoderm, several of the large vessels of which are 
shown in Fig. 9. The central fissure, f, of the placenta is very 
