REPRODUCTION AND DEVELOPMENT 263 



Metagenesis may be regarded as a very general characteristic of liv- 

 ing objects and is one of the many facts that show a real relationship 

 between plant and animal types of protoplasm. 



Parthenogenesis. It was stated previously that in most cases 

 eggs require the activating influence of the sperm cell in order to 

 initiate and undergo development into an adult individual. The 

 eggs of some animals, however, normally develop without being 

 fertilized. This phenomenon is known as parthenogenesis. Clear 

 examples are to be found in the life cycles of some insects. For 

 illustration, the life cycle of one common species of plant lice, an 

 aphid, may be described. 



The form that winters over is known as the stem mother. In the 

 Spring she lays eggs which develop parthenogenetically. This brood 

 is made up exclusively of females. In turn the eggs of these females 

 are also parthenogenetic and so in the Spring and Summer genera- 

 tions no males occur. In late Summer or early Autumn, however, 

 broods are produced which are of both sexes in approximately 

 equal numbers. Succeeding late broods are derived from fertilized 

 eggs and the stem mother which winters and starts the series of 

 maleless broods the following Spring is herself the result of the 

 development of a fertilized tgg. 



Parthenogenesis normally occurs in some members of the phyla 

 Nemathelminthes and Platyhelminthes and among the class In- 

 secta of the phylum Arthropoda. It may be artificially induced in a 

 variety of eggs. It is accomplished most easily with the ripe eggs of 

 certain Echinodermata (starfish eggs). Appropriate treatment of 

 the eggs with a wide variety of chemical agents, with brief exposure 

 to high temperature, with hypo- and hypertonic solutions, by me- 

 chanical shaking, and other methods will cause a normal develop- 

 ment of the embryo. Frog eggs have been activated by pricking 

 with a needle first dipped in a salt solution. Adult frogs without any 

 male parent have been reared from such eggs. 



