THE LIFE CYCLE 81 
which has been deposited outside the body of the mother, 
it is suid 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- 
Fic. 39.—First stages in embryonic development of the pond snail (Lymnq@us). a. 
egg cell; 4, first cleavage ; c, second cleavage ; d. third cleavage ; ¢, after numer- 
ous cleavages; f, blastula (in section); g, gastrula, just forming (in section) ; 
h, gastrula, completed (in section).—After RaBu. 
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, 2). 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 
