THE PLACENTATION OF PERAMELES. 405 



capillaries, and thus takes no part in the constitution of the 

 functional placenta. 



{b) Allantois and Mesenchyme of AUanto-cho- 

 rion. — Tlie allantoic stalk is ovalish in cross-section (fig. 10), 

 measuring in long diameter '3 mm., and in short '18 mm. In 

 it run the three allantoic vessels, two smaller arteries {all. a.), 

 and a larger vein {all. v.). The vessels are lined by a delicate 

 endothelium, round which the mesoderm is condensed to form 

 a thin sheath. Lying in the mesenchyme between the two 

 arteries is the small canal of the stalk {all. cl.) lined by a layer 

 of somewhat flattened entoderm. The stalk is covered ex- 

 ternally by a layer of mesothelium, internal to which and 

 forming the matrix of the stalk are branched mesenchyme 

 cells. 



The stalk enters the body of the embryo behind and to the 

 right of the intestinal loop to which the yolk-stalk is attached, 

 and has the usual relations ; i. e. the stalk, now consisting of the 

 entodermal canal and the two arteries, passes back in the 

 median line attached to the inner surface of the ventral abdo- 

 minal wall, and its canal finally opens into the cloaca. In the 

 abdomen and close to the body of the embryo, the entodermal 

 lining of the canal consists of a low cubical epithelium. At 

 its distal end the stalk expands into the flattened vesicular 

 portion of the allantois, the canal of the stalk being in direct 

 continuity with the cavity of the vesicular portion (fig. 11). 

 This latter is an exceedingly thin-walled sac, possessing a con- 

 tinuous cavity of very irregular form (fig. 7, all. c). The 

 inner or coelomic wall of the allantois (fig. 7, coe. w.) is fairly 

 smooth and unfolded, but its outer wall has grown out into 

 great hollow folds which enter the deep depressions of the 

 uterine surface, and thus is of far greater extent than the inner 

 wall. 



The allantoic cavity is lined by a very thin layer of ento- 

 derm with small flattened or ovalish nuclei (fig. 7, all. ent.). 

 The mesoderm covering the inner wall (fig. 7, cce. w.) consists 

 only of a thin mesothelial layer, except along the vessels. The 

 mesoderm of the outer allantoic wall is now organically con.. 



