MAMMALIA. 



309 



which the other layers grow round. As the embryo is folded 

 off, the yolk-sac comes to form a ventral appendage to its body. 

 A yolk-sac blood-circulation is established, and this appendage 

 becomes much flattened and shaped like an umbrella, with the 

 handle attached to the embryo (Fig. 95). 



The amnion is formed as in the Chick (Fig. 82), but there the 

 outer limbs of the fold contain ectoderm and mesoderm, and, 

 with the ectoderm and somatic mesoderm of the yolk-sac, with 

 which they are continuous, fuse with the vitelline membrane. 



'7.7 



Fig. 95. EMBRYONIC APPENDAGES OF THE RABBIT Diagrammatic longi- 

 tudinal section (from Kolliker, after Bischoff). c, Embryo; a, amnion, 

 and stalk of allantois (al); sh, subzonal membrane; pi, placental villi; 

 fd and ed, vascular and hypoblast layers of yolk-sac; ds, placed in 

 narrow cavity of yolk-sac ; r, space filled with fluid. 



Here the outer limbs of the fold contain ectoderm alone (proba- 

 bly), and are continuous with the ectoderm of the yolk-sac, which 

 splits off. This continuous layer of ectoderm forms a subzonal 

 membrane, lining the zona radiata. To this membrane the flattened 

 yolk-sac attaches itself closely over a large area. 



The allantois (cf. Fig. 82) grows out from the hinder end of 

 the alimentary canal, and becomes closely united with the sub- 

 zonal membrane for a small circular area on the dorsal side of 

 the embryo, to which it remains attached by a narrow stalk. From 

 this area numerous slender processes, villi, grow out, which fit 



