THE PLACENTATION OF PEEAMELES. 



395 



The periphery of this true chorionic area thus indicates the 

 limit of extension of the coelora, and beyond that limit chorion 

 and iuvaginated yolk splanchuopleure alike merge into the 



amn. 



coe. 



vase. ompk. 



proa.n 



s.t. ) 



bilomph. y.spl. 



Diagram showing tlie arraugeiueut of the foetal membranes in Perameles. 

 amn. Amnion, all. c. Allantoic cavity, all. tnes. Allanto-cliorionic me- 

 senchyme, all. s. Allantoic stalk. 6il. omph. Bilaminar omplialopleure. 

 ch. Marginal zone of true chorion around the allanto-cliorionic area. coe. 

 Extra-embryonic splanchnocoele. coe. to. Inner or ccelomic wall of allan- 

 tois. proa. r. Persistent remnant of proamnion, s. t. Sinus terminalis. 

 vase. omph. Vascular omphalopleure. y. c. Cavity of yolk-sac. t/. spl. 

 Iuvaginated yolk-sac splanchuopleure. The ectoderm is represented by 

 a thin line, the entoderm by a dotted line, and the mesoderm by a thick 

 line. 



unsplit wall of the blastodermic vesicle or primitive yolk 

 cavity (text fig., vase. omph.). For this outer unsplit wall of 

 the yolk-sac no very suitable name exists in the literature of 

 embryology. Semon simply terms it " ausseres Blatt.'^ If 

 we restrict the use of the term chorion, as Minot has done, 

 then such terms as "omphalo-chorion " (Fleischmann), " Dot- 



