THE LIFE CYCLE 83 
occurs in the development of all or nearly all many-cclled 
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, 
a 
Fia. 40.—Honey-bee. a, adult worker ; 5, 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. 
