PLACENTA IN PEKAMBLBS 149 



following account only with those features which seem to be 

 peculiar to the Tasmanian form. 



The Allantois. — This agrees in all respects with that of 

 other species. 



In the vesicle the distinction between the outer or placental 

 wall of the allantoic vesicle and the inner or coelomic wall is 

 obvious. A feature w^orthy of note is the somewhat dense look 

 of the allantoic vesicle in surface view. This is due to the thick- 

 ness of its walls, the mesodermal layer of which is much denser 

 and thicker than in the species examined by Hill. 



The Vascular Omph alopleure. — The vascular om- 

 phalopleure (fig. 23, r. omph.) is essentially similar in structure to 

 that of other species. It is, however, as well to point out that 

 the tenuity of the ectodermal layer of this area, on which Hill 

 laid so much stress, is not so pronounced a feature in P . g u n n i 

 as in the forms examined by him. It is, however, still much 

 thinner than in Didelphys (Selenka, 1886-7). In P. gunni 

 the ectoderm cells are somewhat columnar with ovalish nuclei, 

 basally situated. The free end of the cell-body is frayed out into 

 pseudopodia-like processes. In the vascular omphalopleure 

 is contained a large proportion of the extra-embryonal vascular 

 system which in the stage under discussion forms a vascular 

 absorptive organ of some complexity and size. No marsupial 

 so far described appears to have a yolk-sac circulation of 

 such an elaborate type as is characteristic of this stage of 

 P . gunni. 



Bilaminar Omphalopleure (fig. 23, hil. omph.). — This 

 of course consists of two layers, trophoblastic ectoderm and 

 yolk-sac entoderm. The histological details of a portion of 

 this wall are shown in fig. 22. It will be seen that the ecto- 

 dermal layer consists of cells which are large and are remarkable 

 for the immense amount of vacuolation which occurs in their 

 cytoplasm. 



These vacuoles are of all sizes occupying in the aggregate 

 almost the whole of the interior of each cell, separated from one 

 another by thin bridges of protoplasm in a special condensa- 

 tion of which the nucleus is contained. The outer ends of the 



