428 THE REPRODUCTIVE FUNCTIONS.. 



vessels, spread over the membrane of the egg, and receiving 

 the influence of atmospheric oxygen through the substance 

 of the shell, which is sufficiently porous to transmit it; and 

 these vessels, being brought into communication with the 

 circulatory system of the chick, convey to its blood this vi- 

 vifying agent. As the lungs cannot come into use till after 

 the bird is emancipated from its prison, and as it was suffi- 

 cient that they be in readiness at that epoch, these organs 

 are among the last which are constructed: and as the me- 

 chanism of respiration in this class of animals does not re- 

 quire the play of the diaphragm, this muscular partition, 

 though begun, is not completed, and there is no separation 

 between the cavities of the thorax and the abdomen. 



The succession of organic metamorphoses is equally re- 

 markable in the formation of the diversified apparatus for 

 aeration, which is required to be greatly modified, at differ- 

 ent 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 slate, 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, corre- 

 sponding to the differences in the modes of aeration em- 

 ployed at different stages of development. In the primeval 

 conditions this function is always analogous to that of aqua- 

 tic animals, and requires for its performance only the sim- 

 pler 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 compart- 

 ments, corresponding to the double function it will have to 

 perform. For this purpose a partition wall must be built 

 in its cavity; and this wall is accordingly begun around the 

 interior circumference of the ventricle, and is gradually car- 

 ried on towards the centre, there being, for a time, an aper- 



