EMBRYOLOGY OP THE MARSUPIALIA. 3 



greatest length 12"5 mm. and in head length 6'5 mm. Its 

 structural characteristics are given in the Appendix. 



Both uteri were submitted to macroscopical examination, 

 while the right uterus and its embryo were alone examined 

 microscopically. 



FcETAL Membranes. — In fig. 1, PI. 1, the embryo is 

 shown in situ in the right uterus, the ventral wall o£ which 

 has been removed. It was noticed at once that the yolk-sac 

 wall was not even slightly adherent to the surface of the 

 mucosa. Whether this non-adherence is due to post-mortem 

 change or to natural causes I am unable to decide. It is 

 quite possibly due to the latter, since the yolk-sac placenta is 

 now in a largely degenerated condition. 



In the figure, the vasculur omphalopleure and the yolk-sac 

 splanchnopleure, which were cut through along their line of 

 attachment to the margin of the allantoic placental area, have 

 been reflected outwards. The embryo is seen lying on its 

 right side, with its head end next the anterior end of the 

 uterus, and its long axis in the antero-posterior axis of the 

 same. In the left uterus the head of the embryo lay next 

 the posterior end of the uterus, so that its position is an 

 inconstant one. The embryo is situated, of course, in the 

 extra embryonic splanchnoccele, which has been opened into 

 by the reflection of the yolk-sac splanchnopleure, and is 

 invested by the amnion. The latter is reflected from the 

 distal end of the thick stalk, which arises from' the mid-region 

 of the ventral abdominal wall. This stalk — the umbilical 

 stalk (fig. 1, u. s.) — is a tubular prolongation of the abdo- 

 minal wall, which encloses a portion of the coelom, and the 

 upper ends of the allantoic and yolk-stalks (fig. 4, PI. 1). 

 From the yolk-stalk the yolk-sac splanchnopleure is I'e fleeted 

 closely round the embryo. It becomes continuous with the 

 vascular omphalopleure, along the line of union of the latter 

 with the marginal zone of chorion which is present round the 

 allantoic placental area. 



The allantoic stalk (fig. 1, all. s.), on leaving the umbilical 

 stalk, passes outwards and forwards, and then, curving 



