REPRODUCTION AND EARLY DEVELOPMENT. 105 



developing embryo is retained in the body of the mother 

 and nourished by her for a considerable length of 

 time. 



In eggs in which there is much food, as in the birds, 

 the embryo develops on this food, and may get well on 

 toward the adult condition before it breaks through the 

 shell and becomes independent. 



If there is little food, the early divisions produce cells 

 nearly equal in size; if there is much food, these first 

 daughter cells may be very unequal (Fig. 20). These 

 inequalities have much to do with the later method of 

 growth and differentiation. 



in. Hatching and Birth. Most eggs w r hich develop 

 outside the body of the mother are surrounded by a more 

 or less definite membrane or shell for their protection. 

 The early stages of division and change take place within 

 this covering. When further growth demands that 

 the young shall be free, this is burst and the embryo is 

 said to hatch. The term birth is applied if the early 

 development is within the body of the mother and de- 

 scribes the passage to the new life outside. The degree 

 of maturity at birth or at hatching is not at all the same 

 in different species. 



112. Direct and Indirect Development. If the embryo 

 hatches into the miniature of the adult, as in the chicken, 

 the development is said to be direct. If, on the contrary, 

 it hatches into some intermediate stage not having the 

 adult appearance and organs, and later undergoes another 

 change into the adult form, it is said to pass through a 

 metamorphosis. This is true of the butterfly, the bee, 

 and many of the insects, beside representatives of several 



