THE LIFE CYCLE 



83 



occurs in the development of all or nearly all many-celled 

 animals. 



51. Continuity of development. In the case of a few of 

 the simple many-celled animals the embryo hatches that 

 is, issues from the egg at the time of or very soon after 

 reaching the gastrula stage. In the higher animals, how- 

 ever, development goes on within the egg or within the 

 body of the mother until the embryo becomes a complex 

 body, composed of many various tissues and organs. Al- 

 most all the development may take place within the egg, 



FIG. 40. Honey-bee, a, adult worker ; b, young or larval worker. 



so that when the young animal hatches there is necessary 

 little more than a rapid growth and increase of size to 

 make it a fully developed, mature animal. This is the case 

 with the birds : a chicken just hatched has most of the 

 tissues and organs of a full-grown fowl, and is simply a 

 little hen. But in the case of other animals the young 

 hatches from the egg before it has reached such an ad- 

 vanced stage of development ; a young star-fish or young 

 crab or young honey-bee (Fig. 40) just hatched looks very 

 different from its parent. It has yet a great deal of devel- 

 opment to undergo before it reaches the structural condi- 

 tion of a fully developed and fully grown star-fish or crab 

 or bee. Thus the development of some animals is almost 



the blastoderm. Our knowledge of gastrulation and the gastrula stage 

 is yet far from complete. 



