658 H. CALDWELL. 



membrane which I found in the uterus of an Ornithorhynchus 

 whose young had just been born^ that the Monotremes possess 

 a somewhat similar arrangement to the condition described 

 above in Didelphia. 



Before this is published I shall have obtained the early 

 stages of Ornithorhynchus, but I have decided to send home 

 this communication without waiting, because of the interest 

 which attaches to the discovery of the exact relations of the 

 embryo to the maternal parts in these Mammalia. 



The facts above described, so far as I see at present, throw 

 little light on the evolution of the placenta in Monodelphia. 

 The arrangement in the Didelphia is a unique one. I shall 

 defer the discussion of it until my future papers on the deve- 

 lopment of the embryo itself. 



DESCRIPTION OF PLATE XLIII, 



Illustrating Mr. H. Caldwell's Paper " On the Arrangement 

 of the Embryonic Membranes in Marsupial Animals." 



Fig. l.^Advanced embryo of Phascolarctos cinereus, removed from 

 the uterus by slight maceration to show the relation of the embryo to its 

 membranes. Natural size. s. z. Subzonal membrane, am. Amnion, al. 

 AUantois. y.s. Yolk-sac (umbilical vesicle), s.t. Sinus terminalis. The 

 mesoblast forming the vascular area of the yolk-sac and allantois is indicated 

 by the red line. 



Fig. 2. — Section through a portion of the wall of an advanced vesicle of 

 Halmaturus ruficoUis in the region of the sinus terminalis, showing the 

 transition from the flat cells of the subzonal membrane covering the vascular 

 area to the amoeboid attaching cells covering the non-vascular area of the 

 yolk-sac. a. Subzonal membrane. h. Hypoblast of yolk-sac. s. t. Sinus 

 terminalis. c. Blood-vessels, p. Blood-corpuscles, amh. Amoeboid cells, 

 with processes torn out of uterine epithelium. /. Flat cells. Zeiss, oc. 3, 

 Obj. D. 



