EMBRYONIC MEMBEANES IN MARSUPIAL ANIMALS. 657 



space. In this way the embryo comes to hang in the interior 

 of a vesicle whose outer wall is the subzonal membrane. The 

 vesicle is oval and attains a size of 35 mm. in its longest, and 

 35 mm. in its shortest diameter. The yolk-sac occupies the 

 greater part of the inner surface of the subzonal membrane. 



Vascular area of yolk-sac. — The allantois in its greatest 

 development occupies an area of about 12 mm. diameter, and 

 in the later stages becomes vascular, but never develops villi. 

 Professor Owen long ago describes the main features of the 

 vascular supply of both yolk-sac and allantois. The whole 

 vascular area is covered by flat cells of the subzonal membrane 

 There is no attachment to the uterus in this region. 



Attachment of embryo to uterus. — The large oval 

 vesicle, with the embryo suspended in its centre after attaining a 

 diameter of 12*5 mm,, begins to attach itself to the wall of the 

 uterus. This attachment is caused by the growth of the cells of the 

 subzonal membrane immediately outside the sinus terminalis; 

 the cells of the subzonal membrane begin to enlarge 

 and become amoeboid. They throw out pseudopodia- 

 like process, which fit in between the cells of the 

 uterine epithelium and serve to attach the blasto- 

 dermic vesicle to the uterus. This attachment is 

 entirely non-vascular, and is the sole means by which 

 the vesicle is attached to the uterus. 



Just before birth the vesicle is sharply marked out into two 

 areas, one of which has a smooth glistening surface and corre- 

 sponds to an area of flat epithelium covering the vascular area 

 of the yolk-sac and the vascular allantois ; the other has a 

 white opaque appearance and corresponds to the area of large 

 amoeboid attaching-cells. In Phascolarctos cinereus the 

 attached area is next the opening of the uterus into the 

 vaginal passages. In the kangaroo-like forms the attached area 

 occupies that part of the uterine wall next the opening of the 

 Fallopian tubes into the uterus. 



Last year I was too late to obtain the early stages of either 

 Ornithorhynchus or Echidna, still I think it probable from the 

 structure of the uterine wall and from the appearance of a 



