REPRODUCTION. 113 



sometimes, compound '; that is to say, composed of a number 

 of zooids produced by gemmation ; so that the zoological 

 individual in these cases becomes an extremely complex 

 being. 



These phenomena of so-called "alternation of generations," 

 or " metagenesis," occur in their most striking form amongst 

 the Hydrozoa ; but they occur also amongst some of the 

 intestinal worms (Entozoa), and amongst some of the 

 Tunicata (Molluscoida). 



d. Parthenogenesis. " Parthenogenesis " is the term em- 

 ployed to designate certain singular phenomena, resulting 

 in the production of new individuals by virgin females 

 without the intervention of a male. By Professor Owen, 

 who first employed the term, parthenogenesis is applied 

 also to the processes of gemmation and fission, as exhibited 

 in sexless beings or in virgin females ; but it seems best to 

 consider these phenomena separately. Strictly, the term 

 parthenogenesis ought to be confined to the production of 

 new individuals from virgin females by means of ova, which 

 are enabled to develop themselves without the contact of 

 the male element. The difficulty in this definition is found 

 in framing an exact definition of an ovum, such as will dis- 

 tinguish it from an internal gemma or bud. No body, 

 however, should be called an "ovum" which does not 

 exhibit a germinal vesicle and germinal spot, and which 

 does not exhibit the phenomenon known as segmentation 

 of the yelk. Moreover, ova are almost invariably produced 

 by a special organ, or ovary. 



As examples of parthenogenesis we may take what occurs 

 in plant-lice (Aphides) and in the honey-bee ; but it will be 

 seen that in neither of these cases are the phenomena so 

 unequivocal, or so well ascertained, as to justify a positive 

 assertion that they are truly referable to parthenogenesis in 

 the above restricted sense of the term. 



The Aphides, or plant-lice (fig. 34), which are so com- 

 monly found parasitic upon plants, are seen towards the 



H 



