38 MANUAL OF ZOOLOGY. 



the name "alternation of generations " was applied by Steen- 

 strup ; but the name is not an appropriate one, since the pro- 

 cess is truly an alternation of generation with gemmation or 

 fission. The only generative act takes place in the reproduc- 

 tive zooid, and the production of this from the nutritive zooid 

 is a process of gemmation or fission, and not a process of gen- 

 eration. The " individual," in fact, in all these cases must be 

 looked upon as a double being composed of two factors, both 

 of which lead more or less completely independent lives, the 

 one being devoted to nutrition, the other to reproduction. The 

 generative being, however, is in many cases not at first able to 

 mature the sexual elements, and is therefore provided with the 

 means necessary for its growth and nourishment as an inde- 

 pendent organism. It must also be remembered that the 

 nutritive half of the " individual " is usually, and the generative 

 half sometimes, compound that is to say, composed of a num- 

 ber of zooids produced by continuous gemmation ; so that the 

 zoological individual in these cases becomes an extremely com- 

 plex 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 in- 

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

 (Molluscoida). 



d. Parthenogenesis. "Parthenogenesis" is the term employed 

 to designate certain singular phenomena, resulting in the pro- 

 duction of new individuals by virgin females without the inter- 

 vention of a male. By Professor Owen, who first employed the 

 term, parthenogenesis is applied also to the processes of gem- 

 mation and fission, as exhibited in sexless beings or in virgin 

 females ; but it seems best to consider these phenomena sepa- 

 rately. 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 with- 

 out 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 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 the cases of 

 the Plant-lice (Aphides), the Honey-bee, and certain Crustacea; 

 tnough in the case of the first of these it is possible that the 



