ALTERNATION OF GENERATIONS. 49 



2. From these ova viviparous, and, in the great majority of 

 cases, apterous forms proceed. 



o. Tlie broods to wliich these give rise are either winged or 

 ppterous, or both. 



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

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

 ture and the supply of food. 



5. On the setting in of cold weather, 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 apterous. 



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 ma}' co-exist with 

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

 tions, xxii, p. 198.) 



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

 Greek o, without ; game^ 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. luitil the germ {psendovum) of 

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

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

 is different from that of the ovum. The 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 ; within it the pseudovum 

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

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



"Although, then, the pseudovum and the ovum of Aphis 

 are exceedingly similar in structure for some time after they 

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

 bo said that the sole difference between them is, that the one 

 re<iuires fecundation and the other not. When the ovum is of 

 the size of a pseudovum which is about to develop into an em- 

 bryo, and, therefore, long before fecun<lation, it manifests its 

 inherent physiological distinctness b}- 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 



