THE PATH OF EVOLUTION 



corresponds to the allantois and to the chorion in the 

 mammalia. The amuion shows very clearly on the 

 third day. It is evidently derived from a fold of the 

 cicatricula, which envelops the chick after having 

 formed the abdominal cavity. The development now 

 progresses uniformly. The remainder of the yolk is 

 found enclosed within the abdomen as soon as the 

 latter is formed, and serves to nourish the chick for 

 the first twenty-fours after the young bird has 

 escaped from its shell. During incubation respira- 

 tion takes place at first, as before stated, by absorption 

 of oxygen through the air space that is found at the 

 large end of the shell soon after it is laid. After 

 two days' incubation the blood vessels that spread 

 over the inside of the shell absorb directly by endos- 

 mose the oxygen that penetrates through the pores of 

 the shell, the latter becoming more porous as the 

 incubation progresses, the shell becoming more and 

 more brittle proportionally, and at last is readily 

 pierced by the beak of the bird.* 



Thus within three weeks, under the influence of 

 heat, moisture and oxygen, the mass of plasmodium 

 constituting an almost undifferentiated cell the new 

 laid egg rises from the condition of structureless 

 protoplasm into that of a highly organized, living 

 creature, with its wonderful apparatus for the circula- 



* Pierre Larousse. T. XI. P. 1263. 

 383 



