ALTERNATIOX OF GENERATIONS. 49 



2. From these ova viA'iparous, and, in the great majority of 

 cases, apterons forms i)roceecl. 



3. The broods to whieii these give rise are eitlier winged or 

 apterons, or both. 



4. The number of successive broods has no certain limit, but 

 is, so far as we knoAv at present, controlled only by tempera- 

 ture and the sui)ply of food. 



5. On the setting in of cold Aveather, or in some cases on the 

 failure of nourishment, the weather being still warm, males 

 and oviparous females are produced. 



6. The males may be either winged or apterons. 



7. So far as I am aware, there is no proof of the existence 

 of any exception to the law that the oviparous female is apte- 

 rous. 



8. Viviparous Aphides may hybernate, and may co-exist with 

 oviparous females of the same species." (Linnaian Transac- 

 tions, xxii, p. 198.) 



The origin of the viviparous, asexual, or agamic (from the 

 Greek «, without ; (jame, marriage) individual, as it may be 

 more properly called, is, up to a certain stage, the same as 

 that of the true egg, i.e. until the gei-m {psendovvm) of 

 the former is detached from the false ovary (pseudovarium). 

 "From this point onwards, however, the fate of the pseudovum 

 is different from that of the ovum, Tlie former begins at once 

 to be converted into the germ ; the latter accumulates yelk-sub- 

 stance, and changes but little. Both bodies acquire their mem- 

 branous investment rather late; Avithin it the pseudoAum 

 becomes a living larva, Avhile the ovum is impregnated, laid, 

 and remains in a state of rest for a longer or shorter period. 



"Although, then, the pseudoA^um and the ovum of AjMs 

 are exceedingly similar in structure for some time after they 

 have passed out of the condition of indifferent tissue, it cannot 

 be said that the sole difference betAveen them is, that the one 

 requires fecundation and the other not. AVhen the ovum is of 

 the size of a i)seudovum Avhich is about to dcAclop into an em- 

 bryo, and, therefore, long before fecundation, it manifests its 

 inherent physiological distinctness by becoming, not an em- 

 bryo, but an ovum. Up to this period the influence of fecunda- 

 tion has not been felt ; and the production of ova, instead of 

 4 



