THE i'LAOENTATION OF PERAMELES. 397 



cavity, and consistiug of bilaminar omphalopleure ; bctweeu 

 these and ruuuing round the mid region of the whole struc- 

 ture, a somewhat annular zone of vascular omphalopleure. 



The allantois of Perameles consists of a long stalk (text fig., 

 all. s.) and a terminal expanded and rauch-corapressed vesicular 

 portion. The stalk, leaving the embryo immediately behind the 

 yolk-stalk, curves round its right side, and, extending through 

 the extra-embryonic coelom, expands at its distal end to form 

 the flattened vesicular portion. In the stalk the allantoic 

 cavity is reduced to a narrow compressed canal opening dis- 

 tally into the continuous cavity of the vesicular portion [all. c). 

 From the flattened form of this latter we may, for descriptive 

 purposes, term that portion of its wall next the coelom the 

 inner or cceloraic wall {coe. w.), and that turned towards the 

 chorion the outer or placental wall. 



In Stage C the mesoderm of the outer wall of the allantois 

 is found fused with the somatic mesoderm of the discoidal 

 chorionic area {all. mes.), the enlarged ectodermal cells of 

 which are firmly adherent to the vascular maternal syncytium. 

 For the chorion, after the allantois has fused with it, we shall 

 employ the term '' allanto-chorion " (Fleischmann). The 

 allantoic vessels consist of a large vein and two smaller arte- 

 ries. They extend unbranched in the stalk, and, in fact, con- 

 stitute its greater bulk. At its distal end the arteries branch 

 out on the inner wall of the vesicular portion of the allantois, 

 while the vein is there formed by the union of two main factors 

 which accompany the main arterial trunks. The latter branch 

 in a dichotomous manner, each of the larger arterial branches 

 being accompanied by a corresponding venous channel. This 

 arrangement is, however, as Fleischmann (10) has pointed out 

 for the cat (cf. his fig. 7, Taf. iv), confined to the larger trunks ; 

 the finer branchings do not thus correspond. These vessels 

 ramifying on the inner wall of the allantois pass round the 

 margin of the allantoic vesicle into the allanto-chorionic 

 mesenchyme of the outer wall, and there form a rich capillary 

 network corresponding to the so richly developed network 

 described and figured by Semon (8) for Phascolarctus (cf. his 



