232 REPRODUCTION IN PLANTS AND ANIMALS 



tiny heart can be seen beating as early as the second day of 



incubation. After about three weeks of incubation the httle 



chick hatches and emerges in almost the same form as the adult. 



Development of a 

 Mammal. — In most 

 mammals after fertili- 

 zation the egg under- 

 goes development 

 within the body of the 

 mother. The blood 

 vessels, instead of con- 

 necting the embryo 

 with the yolk as in the 

 chick, are attached to 

 an absorbing organ, 

 known as the jplacen'ta. 

 This structure sends 

 branchlike processes 

 into the wall of the 

 u'terus (the organ 



which holds the embryo) and absorbs nourishment and oxygen 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 in a helpless 



condition, usually, and are nourished by 



milk furnished by the mother until they 



are able to take other food. Thus, higher 



in the scale of life, fewer eggs are formed, 



but those few eggs are more carefully pro- 

 tected and cared for by the parents. The 



chances of their growth into adults are 



much greater than when many eggs are 



produced and no care taken of them. 

 Summary. — In this chapter we have 



found two general methods of reproduction, 



At the left is an egg, opened to show the embryo 

 at the center (the small central spot surrounded by a 

 light circle). At the right is an English sparrow one 

 day after hatching. Compare this helpless sparrow 

 with a newly hatched chick. 



The embryo (e) of a rab- 

 bit, and the placenta {v) or 

 absorbing organ ; ct, the 

 umbilical cord connecting 

 the embryo with the pla- 

 centa. (After Haeckel.) 



