AND MORPHOLOGY OF APHIS. 211 



wliich gives rise to the true egg. The pseudovum is detached from the iiseudovarium in 

 the same way as the ovum from the ovarium. In both cases, the act of separation is in 

 every respect a process of gemmation. 



Prom this point onwards, 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 accu- 

 mulates yelk-substance, and changes but little. Both bodies acquke theu* membranous 

 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 pseudovmn and the ovum of Aphis are exceedingly similar in 

 structui'e for some time after they have passed out of the condition of indifferent tissue, 

 it cannot be said that the sole difference between them is, that the one requires fecundation 

 and the other not. When the o'vum is of the size of a pseudo^^Im which is about to 

 developc into an embryo, and therefore long before fecundation, it manifests its inherent 

 physiological distinctness by becoming, not an embryo, but an ovum. Up to this period 

 the influence of fecundation has not been felt ; and the production of ova instead of pseud- 

 ova must depend upon a something impressed upon the constitution of the parent before 

 it was brought forth by its viviparous progenetrix. 



In this resjiect, the ova of Aphis exhibit the same relation to the pseudova as the 

 ephippial eggs of Daphnia (whose development has been so well described by Mr. Lub- 

 bock) bear to the agamic eggs; for the histological change in the ovarium oi Dajjhnia, 

 which precedes the development of the ephippial eggs, is clearly sho^vn by Mr. Lubbock 

 to have no relation to fecundation. 



Let me remark on yet another interesting, though perhaps only partial, analog}^ Von 

 Siebold has shown that the ova of the Queen bee produce females or males, according as 

 tliey are fecundated or not. The fecundated o^iim produces a queen or a neuter accord- 

 ing to the food of the larva and the other conditions to which it . is subjected ; the un- 

 fecundated ovum produces a di'one. Now, what have we seen in Aphis ? The fecim- 

 dated egg produces viviparous Jp)hides, which are the equivalents of the neuter bees ; 

 and from them are eventually produced males and oviparous females. The ovipai-ous 

 females are fecundated and lay eggs which produce only viviparous or neuter Aphides. 



On the view wliich Dr. Carpenter and myself take of the zoological individual, the 

 whole produce of a single fecundated ovum of the Ap)his is as much the Ap)his individual 

 as it is the Bee individual. Consequently we have two equivalent and related series. 



I. II. 



f Ora requiring impregnation, and males. 

 Bee. Imprearnated ova 1 . . f Neuters or females "1 , • I r. i i • i • ■ • 



' ^ > producmg < ^ producnig< Females which give rise to ova requmng 



Apnis. Impregnated ova J I Viviparous neuters J 1 . i- i i 



■^ "^ ° '^ L impregnation, and males. 



The fact that in the one case the males are developed from pseudova resembling fuUy- 

 formed true ova, and in the other from pseudova resembling imperfectly-formed ova, 

 makes no essential difference in the analogy, but only demonstrates stiU more clearly the 

 impossibility of drawing any absolute line of demarcation histologically between ova 

 and buds. 



VOL. XXII. 2 F 



