260 ZOOLOGY. 



blood-vessels. It grows out into the coelom and thence 

 into the space between the true and false amnion a space 

 continuous with the coelom and then spreads out close 

 under the false amnion or serous membrane. In this 

 way only a series of thin membranes and the porous shell 

 are interposed between the air and the blood, and aeration 

 of the latter is readily effected. 



We have now described the essential features of the 

 development of the chick as a whole, and the extra- 

 embryonic structures attached to it. The detailed develop- 

 ment of the several systems will be treated separately a 

 little later. All we need now add is that, when the time 

 for hatching draws near, the yolk-sac, now a tiny remnant, 

 is withdrawn within the body- wall ; the latter closes over 

 and nips across the stalk of the allantois, which dries up 

 and is cast away, and the chick bursts through the true 

 amnion just as it does through the outer membranes and 

 shell. 



