60 



INVEBTEBBATE MOBPHOLOQT. 



genesis. Examples of this phenomenon are to be met with in 

 Insects, a familiar one being the common Hive Bee, the 

 queens of which species deposit large numbers of eggs, 

 those last deposited, which give rise to drones, being unfer- 

 tilized and developing parthenogenetically. In certain flies 

 (Cecidomyia) this parthenogenetic development of the ova 

 may occur while the insect is still in the larval or maggot 

 stage, a phenomenon which is known as poedogenesis (Fig. 29). 

 Alternation of Generations. — The majority of forms which 

 possess the power of non-sexual reproduction also repro- 

 duce by the sexual method, no definite relation 

 existing, however, between the two processes. 

 In some cases, however, a definite relation is 

 established, the one method succeeding the 

 other with rhythmic regularity, the individuals 

 also which reproduce sexually differing materi- 

 ally in form and organization from those which 

 gave rise to them by a non-sexual method ; 

 such a condition of affairs is termed Alter- 

 nation of Generations, a generation of in- 

 dividuals reproducing only by a non-sexual 

 method alternating with a second generation 

 reproducing exclusively or almost so in the 

 sexual manner. Typical examples of this 

 process are afforded by the Discomedusse, 

 in many of which the individual produced 

 by the development of the ovum is a fixed, 

 Fig 29 — p^d cylindrical organism of simple structure, known 

 GBNBTic Gecido- 3'S a polyp, possessing the power of non-sexual 

 myia Labva (after reproduction (see Fig. 55). By a series of 

 ^"^^^^j^"^" ^'""'transverse divisions it gives rise to a linear 

 colony of individuals which in the course of 

 development assume a form very different from that of the 

 parent polyp, becoming more complicated in structure, more 

 highly organized, and free-swimming. These organisms, 

 known as Medusae, are the sexual generation, producing sper- 

 matozoa and ova, the latter after fertilization developing a 

 non-sexual generation, a polyp, with which the cycle begins 

 again. 



