384 THE PHYSIOLOGY OF REPRODUCTION 



placenta. The superficial capillaries of the mucosa, which are 

 slightly dilated, are separated from the vitelline vessels by the 

 uterine epithelium and a thin layer of foetal ectoderm. Through 

 the two layers the gaseous exchange probably takes place. 

 Beyond the sinus terminalis, the non-vascular part of the wall 

 unites over an annular zone with the uterine epithelium by 

 enlarged ectodermal cells. These syncytial " Nahrzellen " are 

 phagocytic, and enclose fragments of epithelium and superficial 

 capillaries. Maternal blood is effused and lies in a space between 

 the ectoderm and entoderm, whence it is transmitted to the 

 cavity of the yolk-sac and serves for nutriment (Hill 1 ). The 

 gestation period is about eight days, as in the opossum. 



In Perameles the placental structures are better developed 

 (Hill 2 ). Before the attachment of the blastocyst, the uterine 

 mucosa undergoes preliminary changes. The capillaries in- 

 crease in size and new vessels are formed ; the interglandular 

 tissue is composed of a loose network of anastomosing cells 

 and the inter-spaces are filled with lymph, the glands increase 

 in length and diameter, and the cells of the surface epithelium 

 lose their boundaries, and fuse to form a syncytium analogous 

 to the symplasma of higher forms (see p. 414). 



Opposite the ccelom, the blastocyst becomes attached to a 

 discoidal area of the uterine symplasma by means of enlarged 

 ectodermal cells, and later its wall is vascularised by the allantois. 

 Outside the disc, the part corresponding to the area vasculosa 

 is also attached by an annular zone, and a yolk-sac placenta is 

 formed. The non-vascular part of the wall is bathed by the 

 uterine fluid as in the opossum (Fig. 80). 



In the discoid area a functional allantoic placenta is de- 

 veloped. The ectodermal giant-cells, like the early tropho- 

 blastic proliferation in Man, disappear, and the allantoic vessels 

 become firmly attached to the symplasma into which the 

 maternal vessels penetrate. A regular interlocking of maternal 

 and foetal tissues is produced, and the two systems of blood- 

 vessels are separated at the most by a thin layer of symplasma. 



1 Hill, "On the Foetal Membranes, &c., of the Native Cat (Dasyurus 

 viverrimis)," Anat. Anz., vol. xviii., 1900. 



2 Hill, "The Placenta! ion of Perameles," Quar. Jour. Micr. Sci., vol. xl., 

 1898. 



