474 HENRY P. OSEORN. 



supply of two arteries and one vein. (4) An amnion closely 

 investing the embryo and reflected over the base of the yolk- 

 sac and allantoic stalks. 



One sees at once that Professor Owen's observations, valu- 

 able as they are, still leave us in doubt as to the real relation- 

 ship existing between the foetal and maternal blood currents, 

 which, after all, is the main question. By what process does 

 the embryo, with little or no food yolk to draw upon, support 

 life during the short but rapid period ^ of intrauterine growth, 

 extending not over seventeen days in the opossum, and thirty- 

 eight days ~ in the kangaroo ? 



My own observations partly confirm and partly contradict 

 those of Professor Owen; they show that not only does the 

 yolk-sac in the Marsupials perform the functions of the allan- 

 tois in the placental Mammals, but that the method is the same, 

 namely, by means of vascular villi developed upon the 

 subzonal membrane over the attached or chorionic 

 portion of the yolk-sac. 



In the early part of March I had the good fortune to receive 

 from one of my students^ a female opossum (Didelphys Vir- 

 giniana), which was found to be in an early stage of preg- 

 nancy. After opening the animal I found that each horn of 

 the uterus had a single swelling an inch and a half long and an 

 inch in diameter. Upon laying one of these open, eight embryos 

 were seen, lying in a row, partly enveloped in one or two long 

 furrows. These furrows would extend along the lower internal 

 wall of each uterus ; if the animal were in its natural position 

 they would then be horizontal — a fact the importance of which 

 will appear later. The foetuses varied considerably in develop- 

 ment, some being nearly twice as large as others. In the 

 larger embryos there were two visceral clefts, the foetal circula- 

 tion was completely established, the fore limb was compara- 

 tively well developed, with the position of the toes faintly 



1 llev. Dr. Bachman, ' Proc. Phila. Acad.,' April, 1848, p. 4G. This writer's 

 statements have been couCrined by several observers. 

 - Owen, ' Comp. Auat. of the Verts,' vol. iii, p. 718. 

 8 Mr. Robert Speir, of South Orange, N. J. 



