GENERATION, SEX AND ONTOGENY 



227 





born ; in both cases the species is effectively maintained. In 

 general, no agency for the perpetuation of the species is so 

 effective as that of care for the young. 



Some animals do not lay eggs, that is, they do not deposit 

 the fertilized egg cell outside of the body, but allow the develop- 

 ment of the new individual to go on inside the body of the 

 mother for a longer or shorter period. The mammals and 

 some other animals have this habit. When such an animal 

 issues from the body of the mother, it is said to be born. When 

 the developing ani- 

 mal issues from an 

 egg which has been 

 deposited outside 

 the body of the 

 mother, it is said 

 to hatch. The ani- 

 mal at birth or at 

 time of hatching is 

 not vet fully devel- 



/ *.' 



oped. Only part 

 of its development 

 or period of im- 

 maturity is passed 



rio. 134. First stages in the embryonic development of 



Within the egg Or the pond snail, Lymnceus: a. Egg cell; b, first cleavage; 



withill the bodv Of c ' seconc ^ cleavage; d, third cleavage; e, after numerous 



, , ^, cleavages; /, blastula in section; g, gastrula just form- 



ing in section; h, gastrula completed in section, 

 part Of its life thus (After Rabl.) 



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 hatching until maturity 

 is reached is called the postembryonic life or postembryonic 

 stages of development. 



The embryonic development is from the beginning up to a 

 certain point practically alike, looked at in its larger aspect, 

 for all the many-celled animals. That is, there are certain 

 principal or constant characteristics of the beginning develop- 

 ment 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. 1346). These 

 two daughter cells next divide so that there are four cells 



