579 
BY JAS. P. HILL. 
Aepyprymnus or Phascolarctus, and thus the portion of the serous 
membrane consisting of ectoderm and somatic mesoderm and 
limiting the extra-embryonic coelom externally forms a discoidal 
area of considerable size. With this discoidal area of the serous 
membrane the allantois fuses, and over it the placental connection 
is established. 
The allantois is well developed and large, and provided with 
an abundant blood supply. It consists of a long and somewhat 
flattened stalk, and a terminal expanded and much flattened 
vesicular portion. The allantoic stalk leaves the embryo immedi- 
ately behind the yolk stalk, bends round the right side of the 
embryo, and extending through the extra-embryonic coelom 
expands at its distal end to form the flattened vesicular portion 
which spreads over the discoidal area of the serous membrane 
above mentioned. In the stalk the allantoic cavity is reduced to 
a narrow compressed canal, appearing in sections as a mere slit, 
lined by greatly flattened entodermal cells. This narrow canal 
opens distally into the cavity of the vesicular portion of the 
allantois, which is likewise lined by a thin layer of flattened 
entoderm. The cavity of the vesicular portion is greatly com- 
pressed by the approximation of the allantoic walls, and in 
sections appears as a long fissure of somewhat varying breadth. 
One can thus readily distinguish two surfaces in this portion of 
the allantois — an inner or coelomic surface and an outer or 
placental surface. 
The mesoderm of the outer surface of the allantois is fused with 
the mesoderm of the serous membrane so that one can no longer 
distinguish between the mesenchyme of the allantois and that of 
the serous membrane, and not only so, one can no longer make 
out the ectoderm of the serous membrane as a distinct and inde- 
pendent layer, the mesenchyme on the outer aspect of the allantoic 
cavity apparently standing in direct connection with the uterine 
mucosa. It seems more probable from my preparations that the 
ectoderm of the serous membrane has fused with the uterine 
mucosa than that it has disappeared in its entirety. However, 
my observations on this point are by no means complete, and the 
