32 MANUAL OF ZOOLOGY. 



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

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

 process is truly an alternation of generation with gemmation 

 or fission. The only generative act takes place in the repro- 

 ductive zooid, and the production of this from the nutritive 

 zooid is a process of gemmation or fission, and not a pro- 

 cess of generation. 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 inde- 

 pendent 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 independent 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 number of zooids produced by con- 

 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 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 

 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- 



