262 



ELEMENTS OF BIOLOGY 



Metagenesis in Other Metazoa. A true alternation of sexual 

 and asexual generations is found in various other Metazoa; certain 

 parasitic Nemathelminthes, some of the Platyhelminthes, and in 

 some lower chordates. Metagenesis also occurs in some insects. In 

 some ichneumon flies, a type of insect related to bees, wasps, and 

 ants, the eggs of the female are laid in the larvae of other insects. 

 In some species a single egg in the process of development gives rise 

 to a great number, several hundred, larval parasites in the body of 



G£PM OF 13 INDIVIDUALS 



EGG WALL 



NUTRITIVE ORGAN 



Fig. 177. — Polyembryony in the development of an insect, Platygaster vernaUs, 

 order Hymenoptera, parasitic in the eggs and larvx of the Hessian fly. Sixteen new 

 individuals may hatch from a single fertilized egg; thirteen are shown as early 

 germs in the figure. (After Patterson, after Leiby and Hill.) 



the host larva. There occurs here by multiple divisions of the germ, 

 an agamic reproduction of the developing parasite, a phenomenon 

 known as polyembryony (Fig. 177). Essentially it is a process o£ 

 metagenesis, for a sexual process of reproduction is followed by an 

 asexual fragmentation of the embryo of the new generation. 



Sufficient has been said here to emphasize the fact that alternation 

 of sexual and asexual generations is a very common type of life | 

 cycle in both plants and animals. Plants have developed this type of 

 reproductive phenomenon as a major feature of the highest and 1 

 most complicated plants; animals have continued the sexual type - 

 of reproduction as the method in the most highly developed forms. 



