30 MANUAL OF ZOOLOGY. 



tinuous 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 many of the intes- 

 tinal worms (Entozoa), and amongst some of the Tunicata 

 (Molluscoida). 



d. Parthenogenesis. "Parthenogenesis "is the term employed 

 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 em- 

 ployed 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 distinguish it from an in- 

 ternal 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 unequivo- 

 cal, 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, which are so commonly found 

 parasitic upon plants, are seen towards the close of autumn to 

 consist of male and female individuals. By the sexual union 

 of these true ova are produced, which remain dormant through 

 the winter. At the approach of spring these ova are hatched ; 

 but instead of giving birth to a number of males and females, 

 all the young are of one kind, variously regarded as neuters, 

 virgin females, or hermaphrodites. Whatever their true nature 

 may be, these individuals produce viviparously a brood of 

 young which resemble themselves ; and this second generation, 

 in like manner, produces a third, and so the process may be 

 repeated, for as many as ten or more generations, throughout 

 the summer. When the autumn comes on, however, the vivi- 

 parous Aphides produce in exactly the same manner a final 



