THE ALLANTOIS. 



381 



cavity, and spreads itself out on the inner surface of the 

 e irorion. In many Mammals the allantois becomes so large 

 that it finally surrounds the whole embryo with its other 

 appendages, as a great covering, and extends over the whole 

 inner surface of the egg-membrane. On cutting such an 

 egg, the first thing met with is a large space filled with 



Fig. 136. — Embryo of Dog, twenty-five days old, opened on the ventral side 

 (as in Figs. 134 and 135). Chest and ventral walls have been removed: a, 

 nose-pits; b, eyes ; c, under-jaw (first gill-arch) ; d, second gill-arch ; efg h, 

 heart (e, right, /, left anricle ; g, right, h, left ventricle) ; i, aorta (origin of ) ; 

 kk, liver (in the middle between the two lobes is the cut yelk-vein); I, 

 stomach ; m, intestine ; n, yelk-sac ; 0, primitive kidneys : p, allantois ; q, 

 fore-limbs ; It, hind-limbs. The crooked embryo has been stretched straight. 



