Ap'kil 1, 1888.] 



SCIENCE, 



455 



famous paper of 1880, upon the descent of 

 the Mammals, derived the Marsupials from 

 the Monotremes, and the Placentals from 

 the Marsupials. Other writers have dis- 

 puted this position. Gill had previously 

 united the Marsupials and Placentals as Eu- 

 theria. In 1893 Osborn, upon paleontolog- 

 ical and odontological grounds, considered 

 the Marsupials as a parallel phylum with the 



in Perameles has led him to adopt the 

 ' parallel ' interpretation, deriving both the 

 Placentals and Marsupials from a Protopla- 

 cental stock. According to this interpre- 

 tation, the Marsupials are to be considered 

 in placentation, as in dentition, in a condi- 

 tion of decadence. Thus he says: " In our 

 view, it is unnecessary to trace the placen- 

 tal ancestry of Eatheria back into the mar- 



C/?e. 



vasc.ompft. . 



a/l.mes. 

 ctlLs. 



bU.ompJv. y.spl. 



Diagram showing tbe arrangement of the fcetal membranes in Perameles: amn., Amnion, all. c, Allantoic 

 cavity. aH. mess., Allanto-ohorionic mesenchyme, all. s., Allantoic stalk, hil.omph., Bilaminar omphalo- 

 pleure. ch., Marginal zone of true chorion around the allanto-chorionic area, cce.. Extra-embryonic splanch- 

 nocoele. cos. w., Inner or coelomic wall of allantois. proa, r., Persistent remnant of proamnion, s. t.. Sinus 

 terminalis. lasc. omph., Vascular omphalopleure. y. c, Cavity of yolk-sac. y. spl., Invaginated yolk-sac 

 splanchnopleure. The ectoderm is represented by a thin line; the entoderm by a dotted line, and the meso- 

 derm by a thick line. 



placentals arising from a common stock, and 

 independently differentiated. In a discus- 

 sion of the tooth development of Perameles, 

 Dr. Hill and Professor Wilson, of Sydney, 

 in 1897, advocated the same view. Semon, 

 however, suggested, in 1896, that the Placen- 

 tals were derived from Marsupials through 

 a Perameles and a Phascolarctus tji'pe, thus 

 supporting Huxley's original position. 

 Hill's study of the placental phenomena 



supial group. The occurrence there of a 

 true allantoic placenta, and its absence in 

 the majority .of members of the order, do, 

 no doubt, at first sight, suggest that in this 

 group we must find the first beginnings of 

 the organ. But we believe that the ex- 

 planation is to be found in the fact that 

 marsupials are, after all, a markedly spe- 

 cialized group, and that in it conditions 

 have obtained producing placental disap- 



