RELATIONS OF OVA AND GEMM/E. 101 



its homological identity established, on structural gr«)unds 

 alone, by means of various transitional forms. 



But the other great character (as generally considered) 

 of bodies homologous with the ova of the higher animals — 

 viz., their dependence on impregnation for their farther de- 

 velopment, would seem, from recent researches, to be tenable 

 only with considerable qualification. The facts now re- 

 corded by several independent and trustworthy observers 

 leave no room to doubt that ova — or bodies undistintruish- 

 able from ova both in structure and relation — do occasion- 

 ally undergo development independently of impregnation. 



§ 2. Hence there may, under certain circumstances, be 

 a difficulty in deciding whether the phenomena are of the 

 nature of gemmation or of ovulation. In fact, one of the 

 cases — that of the Aphides — quoted as an example of gem- 

 mation, in the course of the foregoing remarks on the dif- 

 ferent forms of alternation, has been the subject of much 

 controversy on this very point. The question has been 

 particularly studied by Owen, Cams, Leydig, Burnett, Hux- 

 ley, Lubbock, and Leuckart, but these authors differ in 

 some important particulars. 



It is admitted that there is a general correspondence be- 

 tween the internal organs of generation in the o\iparou.s 

 and viviparous individuals; and though, as Siel)old was the 

 first to point out, there is also an important difference in 

 the absence, in the latter, of the collet erial (/la /ids and 

 spermatheca, yet, as these are organs ministering exclusively 

 to the impregnation and encasing of the eggs, their defi- 

 ciency does not touch on the question of essential identity 

 or otherwise, in the process of their first formation. Tlii.s 

 is distinctly denied by Dr. W. Burnett, but, with some qua- 

 lifications, is rather supported by most of the other ob- 

 servers. 



Burnett maintains that the viviparous Aphides have no 

 proper sex — that they possess no organs, external (►r inter- 



