230 



THE CONTINUITY OF THE RACE 



helplessness that may be shown. To see this one has only to compare 

 the newly born kitten, pup, or mouse or the newly hatched songbird, with 

 equally young calves, colts, fawns, quail, chickens, or ducks. 



In the amphibians and in most of the nonvertebrates, the young at 

 hatching do not at all resemble the parent forms that reproduced them. 

 Instead, they appear as larval forms that must pass through long periods 

 of finding their own food before they are able to transform into the adult 

 form. This transformation of larvae into adults is termed metamorphosis. 

 In many of the lower invertebrates the larval stage appears to be clearly 

 correlated with the small amount of food contained in the egg, a condition 



te 



m* 



Fig. 15.11. Parental care. Ostriches guarding their nest and newly hatched young from wart 

 hogs. (Photo of African group, courtesy American Museum of Natural History.) 



that forces the young to become self-supporting at a very early stage; but 

 in the insects and many other groups, the larval stage is associated with a 

 highly successful type of life cycle in which the larval forms are as highly 

 organized for certain activities as the adult organisms are for others. 



EMBRYONIC DEVELOPMENT 



In the preceding paragraphs we digressed from a consideration of the 

 germ cells to look at some of the relationships and appropriate parental 

 behaviors that result in the fertilization of the egg and the care of the 

 developing young. We shall now return to the fertilized egg or zygote 

 and see something of its subsequent development. 



The period between the fertilization of the egg and the birth or hatch- 

 ing of the young is a time of rapid growth and change. During this 

 period, in which the developing offspring is known as an embryo (Greek, 

 en, "in," and bruo, "bud"), it changes from a one-celled zygote into a 



