DEVELOPMENT OF A MAMMAL 



619 



placenta. 



yellow 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 

 is 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 the development is followed in a series of 

 eggs taken from an incubator at intervals of six hours or less, this 

 spot will be found to increase in size ; and later the little embryo 

 will lie on the surface of the yolk. Still later small blood vessels 

 can be seen reaching into the yolk for food, and the tiny heart can 

 be seen beating as early as the second day of incubation. After 

 about three weeks of incubation the little chick hatches and 

 emerges in almost the same form as the adult. 



Development of a mammal. In most mammals after fertiliza- 

 tion the egg undergoes development within the body of the mother. 

 The blood vessels, in- 

 stead of connecting 

 the embryo with the 

 yolk as in the chick, 

 are attached to an ab- 

 sorbing organ, known 

 as the placenta. This 

 structure sends 

 branchlike processes 

 into the wall of the 

 uterus (the organ 

 which holds the em- 

 bryo) and absorbs 

 nourishment and oxy- 

 gen by diffusion from the blood of the mother. After a length 

 of time, which varies in different species of mammals (from about 

 three weeks in a guinea pig to twenty-two months in an elephant), 

 the young animal leaves the protecting body of the mother, or is 

 born. The young are born, usually, in a helpless condition and 

 are nourished by milk furnished by the mother until they are able 

 to take other food. Higher in the scale of life, fewer eggs are 

 formed, but those few eggs are more carefully protected and cared 

 for by the parents. The chances of becoming adults are much 



..umbilical cord 



..blood vessel, 

 of -parent 



yolk 

 •sac 



fluicC.... 



.embryo 



In a mammal, the embryo is attached by the umbilical cord 

 to the placenta of the parent. Fluids surrounding the embryo 

 in the uterus serve to protect it against shock. 



