THE LIFE CYCLE 



81 



which has been deposited outside the body of the mother, 

 it is said to hatch. The animal at birth or at time of hatch- 

 ing is not yet fully developed. Only part of its development 

 or period of immaturity is passed within the egg or within 

 the body of the mother. That part of its life thus passed 

 within the egg or mother's body is called the embryonic life 

 or embryonic stages of development ; while that period of 

 development or immaturity from the time of birth or hatch- 

 ing until maturity is reached is called the post-embryonic 

 life or post-embryonic stages of development. 



50. First stages in development. The embryonic develop- 

 ment is from the beginning up to a certain point practically 

 identical for all many-celled animals that is, there are cer- 



FIG. 39. First stages in embryonic development of the pond snail (Lymnoeus). a, 

 egg cell ; b, first cleavage ; p, second cleavage ; d, third cleavage ; e, after numer- 

 ous cleavages ; f, blastula (in section) ; g, gastrula, just forming (in section) ; 

 h, gastrula, completed (in section). After RABL. 



tain principal or constant characteristics of the beginning 

 development which are present in the development of all 

 many-celled animals. The first stage or phenomenon of 

 development is the simple fission of the germ cell into 

 halves (Fig. 39, V). These two daughter cells next divide so 

 that there are four cells (Fig. 39, c) ; each of these divides, 

 and this division is repeated until a greater or lesser num- 

 7 



