44 
ANIMAL PHYSIOLOGY. 
Fic. 47.—Shows the embryos of four mammals in the three corresponding sae of a hog (H), 
calf (C), rabbit (R), and a man(M). The conditions of the three different stages of devel- 
opment, which the three cross-rows (I, II, ITI) represent, are selected to correspond as 
exactly as possible. The first, or upper cross-row, I, represents a very early stage, with 
Fao mnnee. and without limbs. The second (middle) cross-row, II, shows a somewhat 
later stage, with the first rudiments of limbs, while the gill-openings are yet retained. 
The third (lowest) cross-row, III, shows a still later stage, with the limbs more developed 
and the gill-openings lost. The membranes and appendages of the embryonic body (the 
amnion, yelk-sac, allantois) are omitted. The whole twelve figures are slightly magnified, 
the upper ones more than the lower. To facilitate the comparison. they are all reduced 
to nearly the same size in the cuts. All the embryos are seen from the left side ; the head 
extremity is above. the tail extremity below ; the arched back turned to the right. The 
letters indicate the same fe in all the twelve figures, namely : v, fore-brain ; z, twixt- 
brain; m, mid-brain ; h, hind-brain; 1, after-brain ; 7, spinal marrow : e, nose; a, eye; 
o, ear ;_k, gill-arches ; g, heart ; w, vertebral column; f, fore-limbs ; 6, hind-limbs ; s, tail. 
(After Haeckel.) 
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