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 yet fully devel- 

 oped. Only part 

 of its development 

 or period of im- 

 maturity IS passed FJG 134 _ Firat ,, tages in the embryonic development of 

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



c, second cleavage; d, third cleavage; e, after numerous 

 cleavages; f, blastula in section; g, gastrula just form- 

 ing in section; h, gastrula completed in section. 

 (After Rabl.) 



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 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 

 t lie simple fission of the germ cell into halves (Fig. 1346). These 

 two daughter cells next divide so that there are four cells 



