4-i 



COMPARATIVK PHYSIOLOGY. 



Fig. 47,— Shows the embryoe of four mammals in the three corresponding stages ; of 

 a hog (H), calf (C), rabbit (R), and a man (M). The conditions of the three differ- 

 ent stages of development, which, the three cross-rows (I, II, III) represent, are 

 selected to correspond as exactly as possible. The first, or upper cross-row, I, 

 represents a very early stage, with gill-openings, 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, Til, 

 shows a still later stage, with the limbs more developed and the gill-openings 



