19 



Development 



An embryo is a living organism in the early unfolding of its form and func- 

 tion. It has potentiality, and its possibilities for the future contrast with what 

 it is at the moment. This is the root of its compelling interest. The two cells 

 which we see through the microscope would not be so unforgettable if we did 

 not know that they were the first ones in the making of a rabbit. They hold the 

 pattern of lifted ears, of still fright and startled leaping, and of rabbits and 

 more rabbits for years to come. 



The development of an embryo is a series of orderly changes in which cells 

 grow and divide and become different. Growth and differentiation are its key 

 processes. Embryonic development may end at hatching or at birth. Birds and 

 other animals that develop and hatch from eggs outside the body are called 

 oviparous. Those that develop from eggs retained in the body are called vivip- 

 arous. These include man and other mammals with such rare exceptions as 

 the duckbilled platypus of AustraHa. 



As cells grow they become larger and heavier. They take in food and from 

 it make chemical substances like their own. By the time a cell is full grown 

 and ready to divide each of its chromosomes has assimilated food and dupli- 

 cated itself in quality and quantity. Multicellular animals grow by controlled 

 increase in cell number as well as cell size. Every human being begins as a 

 single cell, smaller than a pin head, scarcely visible to the naked eye. At birth, 

 nine months later, a baby is an organization of over 200 billion cells and 

 usually weighs about seven pounds. Increases in weight and cell number 

 are controlled and limited; men, mice, and elephants have their respective 

 limits. 



The animal pole of the egg is the most active in physiological exchange with 

 its environment even while the egg is in the ovary. It usually marks the future 

 anterior end of the embryo. Various regions of the fertilized egg are set off by 

 differences in appearance and function (Fig. 19.1). The gray crescent on the 

 surface of the fertilized frog's egg is the scene of great activity, since its posi- 



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