THE BIRD 



247 



portion) to the outside of the body. Before the egg is laid a shell 

 is secreted over its surface. If the fertilized egg of a hen be 

 broken and carefully examined, on the surface of the yolk will be 

 found a little circular disk. This is the beginning of the growth of 

 an embryo chick. If a series of eggs taken from an incubator 

 at periods of twenty-four hours or less apart were examined, this 

 spot would be found at first to increase in size ; later the little 

 embryo would be found lying on the surface. Still later small 

 blood vessels could be made out reaching into the yolk for food, 

 the tiny heart beating as early as the second day of incubation. 

 After about three weeks of incubation the little chick hatches ; 

 that is, breaks the shell, and emerges in almost the same form as 

 the adult. 



Development of a Mammal.- -In mammals after fertilization 

 the egg undergoes development within the body of the mother. 

 Instead of blood vessels 

 connecting the embryo with 

 the yolk as in the chick, 

 here the blood vessels are 

 attached to an absorbing 

 organ, known as the pla- 

 centa. This structure sends 

 branch like processes into 

 the wall of the uterus (the 

 organ which holds the em- 

 bryo) and absorbs nour- 

 ishment and oxygen by 

 osmosis from the blood 

 of the mother. After a 

 length of time which varies 



in different Species of mam- The embryo (e) of a mammal, showing the ab- 



mals (from about three sorbing organ in black, the^ branch-like pro- 



. . . cesses which absorb blood from the mother 



Weeks in a guinea pig to being shown at 0) ; ct, the tube connecting 



twenty-two months in an the embr y with the absorbing organ or 



placenta. 



elephant), the young ani- 

 mal is expelled by muscular contraction of the uterus, or is born. 

 The young, usually, are born in a helpless condition, then nour- 



e. 



