ORGAMC DEVELOPEiMLNT. 549 



sified apparatus for aeration, which is required to 

 be greatly modified, at different periods, in order 

 to adapt it to different elements : of this we have 

 already seen examples in those insects which, after 

 being aquatic in their larva state, emerge from the 

 water when they have acquired wings ; and also in 

 the steps of transition from the tadpole to the frog. 

 But similar, though less conspicuous changes occur 

 in the higher vertebrated animals, during the early 

 periods of their formation, corresponding to the 

 differences in the modes of aeration employed at 

 different stages of developement. In the primeval 

 conditions this function is always analogous to that 

 of aquatic animals, and requires for its performance 

 only the simpler form of heart already described, 

 consisting of a single set of cavities ; but the system 

 being ultimately designed to exercise atmospheric 

 respiration, requires to be gradually adapted to 

 this altered condition ; and the heart of the Bird 

 and the Quadruped must be separated into two 

 compartments, corresponding to the double function 

 it will have to perform. For this purpose a parti- 

 tion wall is built in its cavity ; and this wall is 

 begun around the interior circumference of the 

 ventricle, and is gradually carried on towards the 

 centre ; there being, for a time, an aperture of 

 communication between the right and left cavities ; 

 but this aperture is soon closed, and the ventricle is 

 now effectually divided into two. Next the auricle, 

 which at first was single, becomes double; not, 

 however, by the growth of a partition, but by the 

 folding in of its sides, along a middle line, as if it 

 were encompassed by a cord, which was gradually 

 tightened. In the mean while the partition, which 



