4i8 THE PHYSIOLOGY OF REPRODUCTION 



the ectoderm and entoderm, whence it is transmitted to the cavity 

 of the yolk-sac and serves for nutriment (Hill^). The gestation 

 period is about eight days, as in the opossum. 



In Perameles the placental structures are better developed (Hill ^). 

 Before the attachment of the blastocyst, the uterine mucosa under- 

 goes preUminaay changes. The capillaries increase 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 



biiomph. 



Pig. 104. — Diagram showing the arrangement of the foetal membranes in Dasywriis. 

 (From Hill, "On, the Foetal Membranes, Placentation, and Parturition of 

 the Native Cat {Dasywms viverrinus)," Anat. Ameiff., vol. xviii., 1900. 



amn, Trunk amnion ; all, allantois ; Tyil.omph, bilaminar omphalppleur ; ch, 

 chorion (diplo-trophoblast) ; coe, extra-embryqnic splanchnocele ; proa, 

 proamnion; proa.l, posterior limit of proamnion; s.t, sinus terminalis ; 

 vasc.omph, vascular omphalopleur ; y.c, cavity of yolk-sac ; y.s, yolk-stalk ; 

 y.spl, invaginated yolk-sac splanchnopleur : thje ectoderm is represented by a 

 thin line, the entoderm by a dotted line, and the mesoderm by a thick line. 



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. 445). 



Opposite the coelom, 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 



* HUl, " On the Foetal Membranes, etc., of the Native Cat {Daaywnis viv&r- 

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



'Hill, "The Placentation of Perameles" Qv,ar. Jowr. Micr. Science, vol. xl., 

 1898. 



