579 BY J AS. P. HILL. 



Ae-pyprymnus or Phascolarctus, and thus the portion of the serous 

 membrane consisting of ectoderm and somatic mesoderm and 

 Hmiting the extra-embryonic ccelom 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 



