ONE-CELLED AND MANY-CELLED ANIMALS 45 



Every individual begins as a single cell, and develops and grows 

 from this single cell to its final complex adult condition. The 

 first single cell is called the fertilized egg cell or ovum, and an 

 egg is simply this primary germ cell, or the embryo which 

 develops from it, together with a greater or less amount of 

 yolk (which is food for the germ), enclosed in a membrane or 

 shell. In the case of those animals which do not lay eggs, but 

 give birth to their young in a free condition, the egg, which is 

 kept inside the body of the mother, is usually composed of the 

 germ alone, food being provided the embryo directly from the 

 body of the mother. After the young has reached a certain 

 stage in its development, it leaves the body of the mother and 

 food is provided it by suckling or in some other way. The 

 development of an animal from first germ cell to the time it 

 leaves the body of the mother, if born free, or until it is hatched 

 from an egg, is called its embryonic development; and the 

 development from then on is called the post-embryonic develop- 

 ment. Beginning students of zoology cannot study the em- 

 bryonic development (embryology') of animals readily, but they 

 can in many cases follow the course of the post-embryonic 

 development, and this study will always be interesting and 

 valuable 



It is a kind of study of particular importance to the economic 

 zoologist, because in all attempts to make better uses of animals, 

 or to restrain their injuries, a knowledge of their life history is 

 essential. This life history includes the facts of their develop- 

 ment and the facts of their habits and general behavior both 

 in immature and mature condition. In the case of an injurious 

 insect, for example, the times and place of egg-laying, the 

 character and duration of the immature stages, the time and 

 place of pupation, etc., are all important conditions. A 

 knowledge of these may enable the economic zoologist to hit 

 upon exactly the best means for combating the pest. 



The radical changes or metamorphoses undergone during 

 development by many insects must be taken into account in 

 any consideration of them as possible enemies of man. Young 

 grasshoppers, for example, are wingless and can be captured 

 and killed by simple methods which would be of no use in the 



