GENERATION, SEX AND ONTOGENY 



227 



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

 general, no agency for the perpetuation of the si)ecies is so 

 effective as that of care for the A-onng. 



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. 'Die mammals and 

 some other animals have this habit. When such an anim;'.l 

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

 the developing ani- 

 mal issues from an 

 egg wliich has been 

 deposited outsitle 

 the body of the 

 mother, it is said 

 to hatch. The "ni- 

 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 

 within the egg or 

 within the body of 

 the motlier. 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 j)eriod 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 embr3^onic development is from the beginning uj) to a 

 certain point ]:)ractically alike, looked at in its larger asi)ect, 

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

 principal or constant characteristics of the l)eginning develop- 

 ment w^hich are present in the development of all many-celltHl 

 animals. The first stage or j)hen()men()n of develoj)ment is 

 the simple fission of the germ cell into halves (Fig. I'Mh). These 

 two daughter cells next divide so that there are four cells 



Fig. 134. — First slaves in the embryonic development of 

 the pond snail, Li/innipus: a. Egg cell; b, first cleavage; 

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

 cleavages; /, blustula — in section; g, gastriila just form- 

 ing — in section; h, gastrula completed — in section. 

 (After Rabl.) 



