THE FCETAL MEMBRANES OF THE MARSUPIALS: 

 THE YOLK-SAC PLACENTA IN DIDELPHYS. 



DR. HENRY F. OSBORN, 

 Professor of Comparative Anatomy, Princeton College. 



The observations ^ upon the advanced condition of the foetal 

 membranes of Didclphys Virgiiiiana, which are here recorded, 

 were made during the spring of 1887, before I learned of the 

 complete success attending Selenka's experiments in the artificial 

 rearing of this marsupial. After my own repeated failures in 

 similar experiments, I can admire the perseverance and skill shown 

 by Dr. Selenka, which have finally been so amply rewarded. 

 Through the kindness of Dr. John A. Ryder, I have lately 

 received a copy of Part I. of Selenka's valuable memoir upon 

 the Development of Didelphys^' This terminates with the early 

 history of the Amnion, but from the outline of the whole 

 period of development, given upon page 108, it is evident that 

 Part II. will complete the full developmental history. For this 

 reason I will not enter so thoroughly into the subject as I should 

 otherwise have done, but must embrace the opportunity which 

 new material affords of correcting some speculative views which 

 I advanced in an earlier paper. ^ 



In the spring of 1883 I procured an opossum with the young 

 in the mid-period of uterine development and established the 

 following facts: (i) That while the allantois is still a small, 

 non-vascular sac, the yolk-sac is closely applied, by both sur- 

 faces, to a considerable area of the subzonal membrane, thus form- 

 ing a vascular disc bounded by the sijtiis terminalis, and this 

 disc has a loose attachment to a definite area of the uterine 

 epithelium. (2) That the portion of the subzonal membrane 



* These were first communicated to the Biological section of the American Asso- 

 ciation for the Advancement of Science, August, 1887. 



^ Studien iiber Entwickelungeschichteder Thiere, von Dr. Emil Selenka; 4^ Heft, 

 l*e Halfte, Das Opossum. Wiesbaden, C.W. Kreidel, 1886. 



' Observations upon the Foetal Membranes of the Opossum and other Marsupials, 

 by Henry F. Osborn. — Quart. Jour. Alic. Science, July, 1S83. 



