548 THE REPRODUCTIVE FUNCTIONS. 



or soldering of their opposite edges, formed into a 

 tube,* which, from being at first uniform in dia- 

 meter, afterwards expands into several dilated por- 

 tions, corresponding with the cavities of the stomach, 

 crop, gizzard, &c. into which they are to be con- 

 verted, wdien the time shall come for their active 

 employment. These new organs are, however, 

 even in this their rudimental state, trained to the 

 performance of their proper offices ; receiving into 

 their cavities, through a tube temporarily provided 

 for that purpose, the fluid of the yelk, and prepar- 

 ing nourishment from it. 



In the mean time, early provision is made for 

 the aeration of the fluids by an extensive but tem- 

 porary system of vessels, spread over the membrane 

 of the egg, and receiving the influence of atmos- 

 pheric 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 vivifying agent. As the lungs can- 

 not come into use till after the bird is emancipated 

 from its prison, and as it was sufficient that they 

 should be in readiness at that epoch, these organs 

 are among the last that are constructed ; and as 

 the mechanism of respiration in this class of ani- 

 mals does not require the play of the diaphragm, 

 this muscular partition is only begun, but not com- 

 pleted, and there is no separation between the 

 cavities of the thorax and the abdomen. 



The succession of organic metamorphoses is 

 equally remarkable in the formation of the diver- 



* Wolff is the author of this discovery. 



